When light strikes any coloured object, the object will absorb only the wavelengths that exactly match its own atomic structure and reflect the rest - which is what we see. Turn this around and it is easy to understand how the colour of anything is a clear indication of its atomic structure or, in simple terms, what it is made of. When light strikes the human eye, the wavelengths do so in different ways, influencing our perceptions. In the retina, they are converted into electrical impulses that pass to the hypothalamus, the part of the brain governing our hormones and our endocrine system.
The hypothalamus (with the pituitary) governs:
Water regulation
Sleeping and behavioural patterns
The balance of the autonomic nervous system
Sexual and reproductive functions
Metabolism
Appetite
Body temperature