The pupil is the black opening in the centre of the eye. Light enters through the pupil and goes through the lens, which focuses the image on the retina. The size of the pupil is controlled by muscles. When more light is needed, the pupil is made larger. In brighter light, the pupil is made smaller. The pupil can be compared with the shutter of a camera. It is surrounded by the iris which is the coloured part of the eye.
The lens changes its shape depending on how far away the eye focuses. The focus point is where the eye is focusing on. The light makes the pupil change its size. When it is darker, the pupils will dilate (get bigger) because they need to allow more light into the eye to see. When it is bright, the pupil will constrict (get smaller) to restrict the amount of light there is getting into the eye so we can see. The pupil is normally black in most animals, but in some reptiles, it can be a different colour.
The main reason why we have a pupil is to regulate the light which travels to the retina. The reason why it has no colour is because the light that travels through the pupil is absorbed by tissues at the inside of the eye. In humans, the pupil is round, but in some animals such as cats, it is shaped like a slit. The pupil size is controlled by a muscle in our eyes.