> What do you think a negative red light source means?
It means that the subject turned a dial to add red light to the color being matched.
Basically, you have an unknown color C, and then an R+G+B color. Sometimes, you can’t match it, so you try matching C+R = G+B. This results in “negative” R, because you’re adding R to the other side of the equation.
The same happens with green and blue, but to a lesser extent.