I''ve been giving underpainting a go since i got my oil paints. I want to experiment more with it. www.conceptart.org has a great thread talk,ing about  this and giving tips. Check the thread: http://www.conceptart.org/forums/showthread.php?t=64674. Heres some of the main points, you should check the thread out yourself though: 1. A value painting is done in grays or another "neutral" color like umber, bluish violet or whatever. This is extremely accurate except for being in a funny color. Then glazes are layered onto the piece to build up the real and final color. It's not unusual to have 200-300 seperate applications of glazes in a complex piece. 2. A value painting is done as above, but in the primary complementary color of the piece. For example, if the piece is a sunny city scene with a lot of sun-drenched yellowish buildings, the underpainting would be in deep purple. A greenish outdoor scene would require a red underpainting, and a flaming warthog from hell would require green. The initial glazes would form a luminescent neutral shadow system, and the final strokes would usually be applied more opaquely. 3. In dry brush, especially on a rough painting surface, the initial sketch is blocked in with fairly bright areas of color that are complementary to the final colors desired. The painting is then done rather opaquely and "roughly"--with lots of underpaint spots showing through. This make the painting "sparkle" a bit because the all the areas have complementary spots throughtout--blue sky has orange spots, grass has red undernearth, sunny people have bright lavendar and purple spots, etc. In this case, the underpainting isn't necessarily as accurate as the drawing under the above two examples. 4. A much more extreme version of Number 3 above. This is close to the Pointellist and extreme Impressionist methodology. The final paint is applied rather opaguely, or in layers that gradually get more opaque, but the strong complementaries underneath are left highly exposed to interact with the final colors on top. The visual mixing of these spots can be controlled to give a lot of depth to the painting, creating imagined OPTICAL colors rather than true mixed shades of various real colors. 5. an underpainting can be done as a mid tone, as if you were working on a strongly toned paper with charcoal and white chalk. A layer of paint is put done with the image accurately depicted as a kind of ghosty image. This would approximate the middle tones of the final piece. then darks and lights are applied sparingly to create the final piece. Nice way to do the human figure. -- And a exercise to try! Here's an experiment that anybody can try really fast... Rough pencil a bunch of 2" squares on some board. Fill two with a solid bright green, another two with a deeper darker green, and another with a washy slap of a bright green so there are obvious brush strokes showing. Make sure everything drys. On one of the solid bright greens, wack on some cad red med or something similar (bright red) as if you were doing a water color. Use a lot of med so its fairly transparent in some spots. On the second bright green square, make up some OPAQUE bright red and using a small brush, cover about 85-90% of the green with a random pattern of daubs of red. You want to end up with a red square with just touches of exposed green showing through. Do exactly the same thing to the two squares with the much darker green in them. On the fifth washy-looking green square, take your transparent glaze of red and cover the entire square with red in a washy manner, just as you did to the underpainting. When these all dry, take a good look at them. You will see a completely different effect with each, and if you squint, you'll start to see various little bits that surprise you. The fifth square may seem to vibrate a bit in some areas, and you'll notice areas that are more red, areas that are more green, and areas that are just wierd color. When you squint at this one, you should see a lot more luminosity and depth to the colors. This is the primary benefit of underpainting over solid color. It makes your brain work, which makes the viewing of the art more pleasurable. -- 1. Underpainting Compliment Base Color: A key to the school of painting you are studying is "contrast". Appropriate choice of value, temperature, and saturation/intensity contrasts will help the image retain vibrancy of color and reflection of natural color/tonal light vibration in the eye of the viewer. You are entering into the area called "color theory". Paint and canvas are not as vibrant as light in life. You are studying light and how to create that feeling in the eye that "light actually has" within a painting(which does not because its mud and solvent and not the sun or a candle or a lamp). Your teachers are pointing you down the path of understanding how to "fake" the feeling of natural light in your paintings...and how to use those devices to control your communication and narrative with color composition There are ways in which you can get colors to vibrate as light does. Having complimentary undertones is perhaps the easiest way in which you can get your light and shade to dance visually. This has to do with optical color mixing, which happens every day in front of you when your eyes are open. It has to do with the fact that light and shadow are COMPLIMENTS in all ways. Using an "opposite color underpainting technique" assists you in getting "light compliment theory" accomplished in the image. There is technique in the process of getting strokes of mud and oil to mimic what light actually does in life. Often, light and shadow are not just to be seen as lighter or darker...but unsaturated and saturated...color compliments...warmer and cooler....LOOK FOR THESE THINGS in life if this is your path of study. You will find true subtlety of these theories and at times will even find ways to prove them wrong.  That is what I love about art theory. It is a constant search to prove oneself wrong so we can learn. By having pieces of your underpainting in contrast you can begin to lure they eye around compositionally. You can create focus. You see, your teachers are simply trying to get you to control color...to see. LOOK. 2. If you paint over the entire underpainting then the compliment color underpainting technique can be used along the way so that you can make color choices. It allows the artist to be aware of colors that are brushed on the canvas as they have colors to compare against. However, there will be places where the underpainting color would be used anyway so at that point the only reason to paint over your underpainting is to gain a particular finish to the paint quality and paint surface. It does not nullify the underpainting if you are using it as a guide for accuracy or perfection of your expression. Some artists believed it to be a waste of time, others swore by it as it allowed them to do things they could not do otherwise with the paint. The underpainting is like the scaffold for which the rest of your painting sits. Some artists wouldnt use it as they believed it took from the freshness of the painting process and the loose qualities they appreciated. Most extremely polished paintings use an underpainting process. But, loose images can also do this. Rembrandts rough paintings are still using the underpainting process and they are far from stiff and clean. It all comes down to what you want to accomplish with your paint. Your teachers are teaching a slew of painting theory from different time periods at you. What you are getting is a cross section of art theory from different time (Baroque, 19th century, Impressionist, 20th century illustration). You could study into the bones of the different periods of painting in terms of technique, color, composition, idea, feeling, mood, expression etc. This will allow you to choose which way of working best suits your needs for communication. Now that you are getting into painting 101, dig deeper. 3. Gray is a cool..or a warm...  If you put any other color on it, it will seem to be a color. If you put a green gray stroke down on that gray surface and then put down a violet gray stroke the canvas base would seem like a blue gray..or a yellow gray...depending on what gray you have as your base. If it seems to be a blue in comparison and you put a warmer blue gray down then the painting base will seem like a cooler blue. Do you see where this is going? A gray underpainting or "grisaille" was the foundation for most painting happening in the baroque time period. It was explored prior to that but became one of the primary techniques around the time of rembrandt. The easiest way to explain it is that it was used as a base for warm light paintings. There is a technique developed that requires many oil glazes and washes to be used over the gray underpainting to develop deep transparent shadows. Then, the light is painted on opaquely and this helps to reflect the light off the paint surface and back to the viewer..where the transparent glazes in the shadows draw in light and fill the shadow paint with "fill light". Typically at that time, if the paintings were of a warm light source, the underpainting was gray. If it was a cool light source the underpainting was based in the burnt sienna range. This allows for easy and immediate seperation of temperature between light and dark as you paint...assuming you are paying attention. Another reason to build off middle values (like gray) is that most of the things in your environment when you paint are not pure darks and pure lights. Working from middle values toward light..and toward dark...allow you to save your lightest lights and darkests darks til last more easily in an art school setting. It is simply easier for the eye to judge lights, darks and colors when there is a base down to paint on. There many ways to use an underpainting. These are just a couple. 4. for compositional color unity...i.e. your painting looks like it is all made up of parts from the same color world and atmosphere...you can have your colors sprinkled about throughout the composition. focal areas are where you place all your best contrasts...balanced to work with your color composition. Too many contrasts might make it garish...perhaps...or perhaps not. It all depends on your image and balance of color/tonal composition and light. If your underpainting is the appropriate contrasting color for that area, then you could leave it..or paint over it with the same color if you wanted to hide the underpainting surface...if it is not the right color for that area then you would paint it out or adjust it. It is all relative. You must be the judge of that. -- Watercolor paints are translucent. Guache is opaque. Acrylics and Oil paints have both opaque and translucent colors. Here are some examples (correct me if I'm wrong): Reds Cadmium - Opaque (although I found an acrylic one that was transparent) English Red - Opaque Alizerin Crimson - Transparent Blues Ultramarine - Transparent Phalo - Transparent Cerulean - Opaque Yellows Lemon Yellow - Opaque Cadmium Yellow - Opaque Yellow Ochre - Opaque Naples Yellow - Opaque Aureoline - Transparent Greens Sap Green - Transparent Purple Dio Purple - Transparent Blacks Lamp Black - Transparent Ivory Black - Opaque Whites are all opaque 2. Underpaintings - why they are helpful. First they can make starting a painting easier, and can save paint. Old Master painters have typically used underpaintings to save time and money. You do this by choosing a limited palette to start, and then working in layers. Another big benefit is glazing, which is used to make reflective surfaces, and the appearance of deep, illusionary shadows. 3. Glazing = mixing a color, either transparent or opaque, with a medium to thin it, and then painting over another color so that they may mix optically. Glazing can create effects that pre-mixing cannot, and is great for shadows, watery surfaces, and shiny objects. It is similary to dry-brush only instead of having dry paint only grip the highest fibres of your canvas/surface, creating a "broken color" optical mix, glazing creates an even surface. There are four types of underpaintings that I was taught. 1. Black & White, or just Black underpaintings. You can use the surface of the painting for White if you wish. We used Griffon White which is mixed with Galkyd to speed the drying time. Here you make a black and white image and glaze colors over it. You use transparent ones to make the shadows seem rich, and to make certain parts of the picture pop out. Generally, whatever you glaze will seem closer to you, so backgrounds should have some opaque colors mixed in. Then opaque colors are mixed in for lights and shapes to make them seem more solid. 2. Value Neutral Underpainting - you draw out your image on the canvas, and then paint by numbers the various sections, making sure each color you use is of about the same value. No areas should look lighter or darker than the rest. Imagine taking a photo in photoshop and adjusting the contrast to the left, till the whole image seems grayish. Do that. Then go into each area and add some color with glazing. Again, glaze with transparent for depth of shadows, and opaque for backgrounds and objects that pop out. 3. Dark Ground - such as gray (suggested above) or dark red, both of which were used a lot by old masters. I think we were supposed to work in dark colors first and then cover over with lights, so that they seem on top and volumous - like the bodies in Old Master paintings. Previously, in egg tempera, artists had painted lights first, and then gone over with darks. This new technique plus blending made figures much more illusionary and volumous. 4. Apelles Palette. Apelles was a famous ancient Greek artist. None of his works survive, but he was much studied and admired both in his time, and again in the Renaissance. It was said he could mix any color from just these four colors: Flake White Lamp Black (basically just charcoal and medium) Yellow Ochre Iron Oxide Red (or English Red today) He basically took the primary colors and substituted black for blue. It works pretty well, and many Renaissance, and Baroque masters used this palette as either underpainting or completed pictures. Rembrandt is an example. It's a good way to build your skill and impress others, seeing how close your picture looks to reality with just these colors. Well, that's my two cents. Hope it helps.