If you think that gray is simply a mixture of black and white, you won't be able to take advantage of the subtleties of this tricky decorating color. Gray is considered a neutral color along with brown. It's basically the result of over-combining primary colors; when you do this to an extreme, you end up with black. Gray may be neutral, but it usually has a discernible hue produced by the colors used to mix it.
Warm and Cool Grays
Before mixing gray for your walls or trim, consider the surrounding color scheme. The scheme is a warm one if the predominating colors in the decor are red, yellow, orange or brown, and it's cool if most of the colors are blue, green or violet. You can mix both warm gray and cool gray, depending on the colors you use, but it's best to combine warm gray with warm colors and cool gray with cool ones. Also, before you start mixing, consider how light you want the gray to be, because that also determines the colors you should use to mix it as well as the type of base you should use.
Mixing Black and White
Adding black pigment to white paint is definitely a way to make gray paint, and because the gray you produce this way is neither cool nor warm, you can use it anywhere. Mix gray by adding black pigment or paint to white paint; have a color reference handy to tell you when you've achieved the proper mixture. Add the black in increments of a few drops at a time if you're looking for an off-white shade and in larger increments to get a deep gray. Stir thoroughly before comparing the color to your reference, and add more black as needed until you get the final shade.
Mixing Primary Colors -- Make a Test Batch
When mixed together, the primary colors -- yellow, red and blue -- produce gray or brown; if you mix them to produce gray, the predominating colors determine the tint of the final mixture and whether or not it's a cool or warm color. It helps to make a test batch in a small container, measuring the proportions of pigments you use, and recreate the color in a paint can once you've achieved the proper mixture. Mix the pigments in a clear paint base if you want to produce a deep gray, such as slate, and in a white base to produce lighter, pastel grays.
Fine-Tuning the Color Mixture
The process for making gray begins by mixing blue and red pigments in more-or-less equal proportions to produce purple. Use more blue to keep the mixture cool and more red if you want a warm color. Adding yellow pigment turns the paint gray, and the more yellow you use, the lighter the mixture will become. Keep a color chip nearby so you know when your color blends or matches it. If the gray is too dark, you can tint it by adding white; use black pigment to create a darker shade if it's too light.