Red and blue light are combined in equal amounts. What color is produced? Cyan light shines on a blue filter. What color is the transmitted beam?
Green light on a blue surface. It depends on how pure the light and the color of the surface are. A pure green light that is derived from a spectrum can only reflect green from any colored surface. Similarly a pure blue surface where the pigment absorbs all colors except blue can only reflect blue no matter what color light is shone on it. So if you shine a pure spectral green light on a pure blue surface then all the light will be absorbed and no color will be reflected. A more practical case is where you get your green light by shining a white light through a green filter (or painting a light bulb green). This gives you a light containing a bit of all colors but whose blue, yellow and red are reduced. Similarly most blue surfaces reflect all colors to some degree but they reflect blue better than most. If you shine such a light onto such a surface then the light that comes off will be cyan. But it will only look cyan if the overall illumination is white light. If you are in a room lit by a green light bulb then your eye adapts. If you now look at a blue board then, although the light coming off it is cyan, it will look blue to you. If go combine red and blue light you always get magenta. Cyan shining through a blue filter. The same comments apply as for the green/blue case. If you generate cyan from a spectrum and shine it onto a pure blue filter then nothing gets through. However if you use a normal cyan light that contains a mixture of blue, cyan and green then the light coming through a normal filter will be blue. It will have some green and cyan (along with a tiny bit of red and yellow etc) but most of the light will be blue.
