[Colors](https://images.app.goo.gl/4YGDV36g85XotLey7)
In: Biology
The visible light spectrum ranges from blue to red. Each of these colors has a very specific wavelength which is on the order of one billionth of a meter, or nanometer (nm). The visible spectrum ranges from roughly 350 nm (blue) to 750 nm (red)
A lot of color you see in nature comes from pigments. These are molecules that absorb certain wavelengths of light and reflect others. While chameleons do have pigments in their skin, it is not the main mechanism for coloration. Under their skin, chameleons have cells that contain guanine (one of the 4 building blocks of DNA). These guanine molecules pack together to form a periodic structure with even spacing. When light interacts with these structures, and their spacing matches a particular wavelength in the visible spectrum, it will be reflected off the structure (resulting in the observed color), whereas the other wavelengths are not ([see Brag’s Law](https://en.wikipedia.org/wiki/Bragg%27s_law)). By changing the spacing of these crystals, the chameleon is able to change the color of light being reflected. This is known as [structural coloring](https://en.wikipedia.org/wiki/Structural_coloration) and can also be observed in some fruits and animals such as a peacock’s feathers.
Happy to answer any other questions you may have as I do research in a tangentially related field.
Edit: Fixed for clarity.
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