Light is made up of multiple wavelengths. However the way we PERCIEVE light is due to the 3 (red, green and blue) colour -sensitive pigments in our eyes. It's quite conceivable that a creature with a different physiology would perceive them differently. We perceive a mixture of red and green light as yellow (or brown, depending on how bright the light is) and the whole concept of additive colour mixing which allows us to watch colour TV is purely down to our own physiology.