What is the science behind colors of paints, dies and food coloring ?

Like purple centeries ago was only available to the rich, and was made from mollusks.

What are the properties of different colors?

I also heard that red velvet cake coloration comes from the blood of bugs?
Is red velvet cake made from Bugs?

Red Dye Made From Bugs. FDA requires they list the ingredient caochineal or carmine because some people may be allergic to it. It’s not found in Kosher products because dietary laws prohibit insects or parts in foods. At the end Ted Allen held up a piece of red velvet cake, but some recipes call for beet juice to give it a red color.

Also what colors are the most expensive to produce?