ok well i'm assuming you know about the structure of the eye
when we 'see', we're actually seeing light which is being either reflected or absorbed by objects.
the light we see passes through the cornea, aqueous humour, lens and vitreous humour. these four components refract light. the light then hits the retina, which is made of photoreceptors - rods and cones.
rods detect movement and light,, they cannot distinguish colours. rods contain rhodopsin (or visual purple), which is broken down into its components when we 'see' and then reforms. this makes an electrochemical signal which can be interpreted by the brain.
cones require light to be stimulated and can distinguish between colours. they do this because there are three types of cones - red, blue and green.
the photoreceptors then transmit the received signals to the brain where an image is formed.
i hope this is right; correct me if there's anything odd there.