What part of a molecule is responsible for its color?

The part of a molecule responsible for its color is melanin. Melanin is a natural pigment found in many organisms, primarily in the skin, hair, and eyes, where it plays a crucial role in determining color. It absorbs various wavelengths of light, which contributes to the appearance of color in these tissues. Different types of melanin can produce a range of colors, from brown to black, which is essential for providing protection against UV radiation and contributing to overall pigmentation in living organisms.

While chlorophyll, carotene, and hemoglobin also play significant roles in color within their respective contexts, they primarily function as pigments associated with specific biological processes—chlorophyll in photosynthesis, carotene in plant coloration, and hemoglobin in the transport of oxygen in the blood. Melanin, however, is the most relevant in terms of skin and hair color variation, making it the correct answer based on the context of the question.