If your kitty tends to prefer her red toy over her green one, it's probably the shape she likes -- unless she's partial to the color gray. Cats can see colors to some extent, but they aren't attracted to them. Cats use color to help them spot tiny movements.
The science of seeing color is the same in cats as it is in humans. The eye has cones inside that differentiate color, whether that eye belongs to you or your feline friend. The magic is in the number of cones. You have three types of cones in your eyes that let you see combinations of red, green and blue. Your cat, however, has only two types: green and blue. Her eyes also are less endowed than yours, with fewer cones to help them see colors.
Although cats have only the blue and green cones, that doesn't mean those are the only colors they see. It also doesn't mean they see the colors the same way we do. It takes yellow to make green, so cats can see some shades of yellow, as well. Blue is a component in colors such as purple, meaning your cat's purple prey will look blue instead of gray. Other colors, such as red and pink, look gray or black to your kitty. But while you see bright, clear colors, you cat lives in a muted world. Her colors are duller that what you see, almost like what you might see at twilight, when there's not quite enough light to distinguish all the colors.
Although your kitty is not big on colors, she makes up for it by having excellent night vision -- much better than yours. She has a tapetum, an iridescent membrane, in her eyes, which basically works like a mirror, reflecting tiny amounts of light back to the rest of her eyes. This allows her to see in 20 percent of the light you can. It also makes her eyes look spooky in your photos, as they reflect the light from your camera's flash. Her night vision is advanced, because this gives her a hunting edge, while seeing colors doesn't do much for her ability to catch prey.
Cats can see a bit more color than dogs, which helps your kitty as she uses her most important hunting tool, the ability to see movement. Your kitty is attracted more to movement than color, although the color can help slightly in her ability to detect the movement, as the colors wave or overlap -- such as a blue bird hopping over brown dirt.