Flame colors are produced from the movement of the electrons in the metal ions present in the compounds when evaporated by an open flame. Electrons would start at a ground state, when nothing is energizing it, and move to an excited state when energized. When you heat ions, the electrons gain energy and can jump into any of the empty electron shells at higher energy levels, depending on how much energy a particular electron happens to absorb from the flame. Each of these jumps involves a specific amount of energy being released as light energy, and each corresponds to a particular color. As a result of all these jumps, a spectrum of colored lines will be produced as it goes back down. The color you see will be a combination of all these individual colors.