The Grotthuss-Draper law, sometimes called the first law of photochemistry, says that only light that is absorbed can drive a chemical change. Light that passes straight through a material, or bounces off it, leaves that material as it found it.
It is a simple rule with a big consequence. To ask what a color of light does to the body, you first have to ask what absorbs it, and where.
How we use it
We lean on this rule to keep our language honest. It is why we talk about specific molecules, like cytochrome c oxidase, instead of saying that light works in general. It is also why the color vocabulary of Auric Structuring is declared as symbolism: green for renewal, red for vitality, crystal for coherence. Those meanings are ours. The chemistry belongs to whatever absorbs the light.