Pigment is what gives objects their color. The primary colors of pigment can also be mixed to make different colors. However, mixing colors of pigment is different from mixing light.
You are probably familiar with mixing paints to make different colors. The primary colors of pigment are usually magenta, yellow, and cyan. Red, yellow, and blue are often used to approximate them.
Pigments reflect some colors of light and absorb others. Imagine a pigment that absorbs blue light and reflects green and red light. What color does it look like?