recently i have been thinking about how colors add when you have 2 different light sources of color and put the streams together, such as taking red and green beams and crossing them to make yellow. i was looking at wavelengths to determine if there was a mathematical way to say what color would be produced by the streams of light. however i ran into a problem when i looked at the red+blue = magenta (purple ish). most colors i can see average instead of add together. red and green added together produce yellow. which if you take the wavelengths of red and green, add them, and divide by 2, produce yellows wavelength. blue and green added together produce cyan, and again adding the wavelengths and dividing produce cyan's wavelength. however adding blue and red wavelengths lets say (440nm and 640nm respectively) = 1080/2 = 540 which is green, and that is not possible. the wavelength for magenta is about 390-405nm (roughly) can anyone help with this problem?