Yellow and purple combine to make a neutral shade of gray. Yellow and purple are complementary colors, and complementary colors combine to form neutral shades of gray or brown, depending on the amounts of each color used.
Complementary colors are opposite each other on the color wheel. Separately, they perfectly complement each other, but they cancel the brightness of the other when mixed together. The three complementary color sets are yellow and purple, red and green and blue and orange.
The color wheel is a circle of colors that consists of primary, secondary and tertiary colors. The three primary colors (red, yellow and blue) cannot artificially occur through mixing, but the secondary colors (orange, purple and green) are the result of mixing two primary colors. A primary color mixed with its closest secondary color on the wheel creates a tertiary color.