Chromophore

White light contains every visible wavelength. A molecule carrying a chromophore absorbs some of them and leaves the rest to return to the eye, and what returns is the colour you see. Chlorophyll takes the reds and leaves green. Melanin absorbs across the whole visible range, and more of it toward the shorter wavelengths, which is why what comes back reads brown.

Only a handful of chromophores matter at the surface of skin. Melanin, in the epidermis, is the main one. Haemoglobin, in the vessels of the dermis, is the second, and it exists in two states — carrying oxygen and having given it up — that absorb differently and read as different colours. Carotenoids from food add a yellow note that is easy to miss until you look.

Depth counts as much as identity. Light that reaches a deep absorber has been scattered on the way in and again on the way out, so deep pigment and dermal vessels look diffuse and lean blue, while pigment sitting high in the epidermis looks sharp-edged and brown.

This is the unglamorous reason a red mark and a brown one are two problems rather than two versions of one. Different molecules, at different depths, and acting on one does not act on the other. It is also the principle behind wavelength-selective devices: the wavelength is chosen because a particular chromophore absorbs it. Which mark is which is a question for a clinician, not for a label.