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Our retina is composed of two types of photoreceptor cells: rods and cones. These cells are responsible for detecting light and converting it into an electrical signal that our brain interprets as vision. Rods are more sensitive to light but do not process color, whereas cones are responsible for color vision and can differentiate between different wavelengths of light.

Rods contain a pigment called rhodopsin, which is highly sensitive to light, making them responsible for our night vision and low-light conditions. They respond to the presence or absence of light but do not differentiate between wavelengths of light, meaning they provide only black-and-white vision. The absence of cones or their reduced function in these low-light conditions leads to a lack of color perception known as scotopic vision.

Color blindness, also known as color vision deficiency, occurs due to a lack of or malfunctioning of certain types of cone cells. There are different types of color blindness, depending on which cone cells are affected. The most common form is red-green color blindness wherein individuals have difficulty distinguishing between red and green colors due to either the absence of or malfunctioning of the red or green cone cells. Another form of color blindness is blue-yellow color blindness, which is less common and affects the ability to distinguish between blue and yellow colors. This occurs when the blue cone cells are either absent or not functioning properly. In rare cases, individuals may have total color blindness, also known as achromatopsia, where they are unable to see colors at all because none of the cone cells are functioning properly. In these cases, they rely entirely on rods for vision, which results in a completely black-and-white view of the world.

In conclusion, rods provide only black-and-white vision because they contain the pigment rhodopsin that is sensitive to light but does not differentiate between different wavelengths of light. Cones are responsible for color vision, but their reduced function or absence in certain types of color blindness can result in an impaired ability to distinguish between different colors.