Despite the presence of color-detecting cones, cats have little or no need to distinguish colors. Until fairly recently, cats were widely believed to be colorblind. In tests, however, domestic cats were successfully trained to distinguish blue, green and yellow (but not red). Their light-sensitive vision also allows them to differentiate among several shades of gray.

Because a cat's eyes are designed for keen night vision, its large pupils must constrict to limit the amount of light entering the retina during the day, thereby preventing them from being dazzled. Domestic cats and many smaller wildcat species possess pupils that narrow to a mere slit, allowing them to see well even in extremely bright light. Big cats such as lions, which hunt in daytime, possess pupils that constrict into small, tight circles instead; their night vision is not as acute as that of their nocturnally active cousins.

In low-light situations, both wild and domestic cats will fully dilate their elliptically shaped pupils into almost perfect spheres to harvest the maximum amount of light. Cats also open and close their pupils during periods of stress or confrontation. A fearful cat's pupils will be fully dilated to create a wider field of vision and take in as much of the surroundings as possible, while those of an aggressive cat are considerably constricted as a threat signal.

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