Visible light falls between 380nm (violet) and 780nm (red) on the electromagnetic spectrum, sandwiched between ultraviolet and infrared. White light comprises approximately equal proportions of all the visible wavelengths, and when this shine on or through an object, some wavelengths are absorbed and others are reflected or transmitted. It's the reflected or transmitted light that gives the object its perceived colour. Leaves, for example, are their familiar colour because chlorophyll absorbs light at the blue and red ends of the spectrum and reflects the green part in the middle.
The "temperature" of the light source, measured in Kelvin (K), affects an object's perceived colour. White light, as emitted by the fluorescent lamps in a viewing box or by a photographer's flashlight, has an even distribution of wavelengths, corresponding to a temperature of around 6,000K, and doesn't distort colours. Standard light bulbs, however, emit less light from the blue end of the spectrum, corresponding to a temperature of around 3,000K, and cause objects to appear more yellow.
Humans perceive colour via a layer of light-sensitive cells on the back of the eye called the retina. The key retinal cells are the cones that contain photo-pigments that render them sensitive to red, green or blue light (the other light-sensitive cells, the rods, are only activated in dim light). Light passing through the eye is regulated by the iris and focused by the lens onto the retina, where cones are stimulated by the relevant wavelengths. Signals from the millions of cones are passed via the optic nerve to the brain, which assembles them into a colour image.
