Humans eyes can only detect visible light, we see different wavelenghts within this range as different colours. Most living things can detect wavelenghts in the visible specturm. Not all animals can see different wavelengths within the visible light spectrum as different colours. How they see the light depend on the type of photorecptors they have.We have two different types of photorecptor cells, rods and cones; Rods are more sensitive to light but do not distinguish colours while cones require more light than rods to be stimulated, they all contain pigments which are made up of opsins. Within the visible spectrum, red light has the longest wavelengthand least energy while violet had the shortest and the most energy. If say red light is shone into the eye, the blue and red cones are not stimulated at all, but the red is. The brain interprets this stimulus as blue. Different wavelenghts of light stimulates the three different types of cones to a different degree. Eqaul stimulation is interpreted as 'white'.
Biology class is paying off.
EDIT: oh yeah, and colour blindness is when you are missing photopsins in your cones, or have fewer, which leads to colour weakness. The most common is lackign green or red photopsin which leads to red-green colour blindness. in Some rare cases, a person migth only have one type of cone which results in single colour vision.