Mixing black and white together is a common way to make the color gray. Other colors can be mixed together to make shades of gray with hints of the original colors present.
Learn to Draw and Paint presents several methods to get nuanced shades of gray paint. The primary colors of red, yellow and blue mixed together produce gray. Mixing the secondary colors of orange, violet and green together will also produce gray. Two complementary colors directly across from one another on the color wheel make gray as well. Black can be mixed with any color to neutralize it, with the result being gray with the original color's undertones.