White light strikes the red curtains and is absorbed into the molecules (why its gets warm under light) the molecules are excited when so much energy is just given to them that they have to get rid of it, fast, or they might start to fall apart, much light is converted to heat, some however (depending on the shape and size of the atoms and molecules), is absorbed by the orbiting electrons which raises their energy levels one 'quanta' promoting them to a higher energy state, quickly they drop back again and release the energy again, but they do it in a rythm, rise, drop, rise, drop, the rythm is constant in amplitude and frequency and becomes a pulse of energy waves that correspond to wavelengths of light (sometimes invisible). these wavelengths are spewed out in all directions (as in shining through a red curtain) or in precise directions (as in, bouncing off a mirror) or both (a dirty mirror) and strike other surfaces which do the same process again or strike the back of your eyeballs, where the pulse of light is converted to a pulse of energy which the brain reads as code for colours.

short answer, things you perceive as red are red because they absorb all wavlengths of light and emit a little in response. what they emit is what you get in your eyes.

hope that makes some sense, its clearer to me now ive explained it, so thanks to you for that.