“Causes of Colors” website is an excellent resource about colors and light. I’ll put it into photography section, since photography is the most obvious application for light and because they have a lot of excellent photographs up there. But everyone should look through that site. It is fascinating.
Why are things colored? There are three causes. Repeated over and over, in innumerable variety, they create our colorful world. Light is made in the yellow glow of candle. Light is lost when light filters through stained glass. Light is moved when sky turns crimson sunset.