# How Negative Color Value is accepted in Tristimulus Values for Mixing Colors?

I read a chapter about Trichromatic Theory of Color Mixture (Yao Wang, et all. 2001), about how we can produce most colors by mixing 3 primary colors. And the amount of three primary colors required to match the target color is called Tristimulus values. And we can produce a color by mixing primaries following this equation :

$$C = { \sum_{k=1,2,3} T_k C_k },$$ where :

• C = Given color
• Ck, k = 1,2,3, represents the color of primary sources
• Tk = Amount of primary colors required to match the color (tristimulus value)

I believe in Trichromatic color theory related to human vision system, primary colors depend on the source, either lights (perceived wavelength, ranged from 380nm-780nm) or digital color (RGB ranged from 0-255), their value can't be negative.

The book mentioned :

"In general, some of Tristimulus value can be negative."

My question is, if primary color can't be negative, how is it that tristimulus value which is the amount of primary color to make another color can be negative ?