The photoreceptor inside retina is the receptor of light that makes an organism capable to see. The photoreceptor consists of cone cells that can distinguish color and rod cells that can distinguish color as black and white image.
There are three kinds of cone cells, red, green and blue. The interaction of stimulus given by these 3 kinds of cone cells gives the perception of color. In color-blind patient, some of the cone cells are damaged or not formed. Losing the cone cells will cause color blind patient unable to perceive color. To have a good visual acuity, you don't need all the cone cells variety, only one kind is enough.
Depending on what you've been learning in bio so far, the glucose is used in a process called glycolysis, which basically turns the glucose into ATP (Energy) which the cell can use.
