At the back of the eye is a small section called the fovea. In this there are lots of cones which are used to distinguish colours. There are three types of cone; short, medium and long which react to blue, green and red, respectively. The fovea has a 2 degree angle which is why colour experiments are carried out using the angle so it excited only the cones. If you go beyond this, you excite the rods which are used more for illumination. So, if you are looking at a red light in a dark space, you will be exciting the long wavelength cones as well as the rods to help you see the colour more clearly and let more light in to the eye.
