Gamut mapping is a method for choosing colors that...limits your choices. A limited palette is actually a valuable painting tool.
Why Limit My Palette?
#1: Unity. A single collection of colors used throughout a painting will contribute to its unity value. Your eyes will pick up on the fact that all the colors are related. This doesn't mean a gamut-planned painting will look mono- or bi- chromatic. More on that later.
#2: Character. You can choose your gamut to be, say, on the warm side of the wheel. Any subject that you paint using that gamut will have a warm quality.
#3: Convenience. If you plan your palette before you paint, you'll rarely have to stop and improvise. Every color you need is already right there, and it's already guaranteed to fit into the scheme of the painting.
I confess that my experience with this technique is limited compared to some other notable artists.
I encourage you to visit James Gurney's site or check out his book if you want in-depth color theory.
Step 1: First, Some Examples:
He must have used most of the color wheel in the scene...
....nope! That lightened triangle in the second photo represents all the hues Rubens used.
Notice that the triangle doesn't include proper blues or greens, yet the foliage appears green and the princess's dress appears blue. They are only blue and green in relation to the rest of the colors, because they are the "bluest" and "greenest" of the group.
So you don't need the whole wheel, even if your scene seems colorful in your head. Remember, limits will help concentrate the effect of a painting and lend it a finished quality, as well as making the application of paint easier.
This works in photography, too. Frodo looks adorable leaving Middle Earth forever. His eyes are blue, his hair is dark and shot with sun, and his elven cloak is a mystical grey-green. Yet the image uses only a tiny gamut.
Software I used to get the gamuts:
Cristian Romero developed a great gamut-detecting program that is available here: http://kauel.com/kgamut/kgamut.zip (free!)
Step 2: Download Color Wheel & Open in Photoshop
Photoshop Elements, Gimp, and most other photo manipulation software will work.
I used Photoshop because I like it and it was on the computers at TechShop. Yes, I made a digital gamut mask at Techshop.
Tip: Techshop is digital-painter friendly. Their computers are top-notch and they have great software (Autodesk, Photoshop...not Corel Painter). But more importantly, they welcome the use of Wacom tablets. They'll install the driver software and help you get set up.
*Yurmby is the nickname given to Yellow/Red/Magenta/Blue/Cyan/Green (YRMBCG) color wheels. A less accurate and useful wheel is the "Standard" Red/Yellow/Blue wheel, which spaces the primaries wrong. For more info, see the intro link to James Gurney.
Step 3: Map the Gamut!
Does it have to be a triangle?
No...check out the examples above. Different gamut shapes will give you different color choices.
You can make it fat, or long, or big, or small. You can even try circles!
Just remember that the goal of gamut mapping is to focus your color choices.
Left-click inside the dotted lines and select "Layer via Cut."
Step 4: Pick Colors From the Gamut
From there, use the eyedropper to pull colors from the gamut and put them in your palette.
-Make sure to pick from the most extreme corners of the gamut. These colors are your "gamut primaries"
-Also pick distinct colors. I picked the "greens" on the left because they are the greenest I'll get within this gamut.
Step 5: Use and Final Thoughts
So, can you make the colors in your palette lighter and darker? Of course; that's a big part of painting.
The problem with mixing "tints" (colors mixed with white) and "shades" (mixed with black) in digital programs is that you can add saturation without meaning to.
Say I want a lighter yellow. Well, I just open the color picker and move the cursor until it's a lighter yellow, right?
No...because I could end up with a yellow that is more saturated as well as lighter. To stay within the gamut, I have to keep my colors at the same saturation level. Think about the difference between lemon yellow and pastel yellow. The lemon's yellow isn't as "watered down" as the pastel yellow, even though they may have the same lightness value.
So be careful that when you're making lighter or darker versions of your gamut colors you keep the saturation levels about the same.
What about oil/acrylic/watercolor painting?
Gamuts aren't just for digital coloring! If you work with real paint, you can make gamuts digitally for planning purposes. You can even make analog gamuts from your paint by painting a color wheel and laying masks over it.
Good luck painting!