So in the back of your eyeballs are special photo receptive bits called cones that you use to see color. There are three different kinds and they each detect red, green, or blue light. Using those three sets of wavelengths and the overlap between them your brain combines which cones are receiving light to make up all the other colors.

You might notice that pixels use those same colors. Once you back up enough it looks like each set of three pixels is actually a single point of light, so your brain combines the wavelengths into a single color.