im reading some of feynman's lecture notes, and i stumbled upon a statement which i do not understand... he says that we can use ANY three different colors to produce any other color. lets say we want to see the color X, and we use yellow blue and red (Y,B,R) then X = yY + bB + rR. where y,b and are are the color coefficients. inorder to produce some colors (in my example lets say we want to get green) we would need to use a negative color coefficient. i.e. the r should be negative while y and b have some positive value to produce green. mathematically i can see it is so, but i couldn't understand how you practically make it... i'll give you the original quote: now, what does he mean when he says "to add them to the other side"? does he mean that if i use three flashlights with red, yellow and blue colors, we have to use the red flashlight from one side of a sheet and the other two from the other side? doesn't sound right to me... can someone explain what he means? how can i produce green in the lab using red, yellow and blue flashlights?