The color of a laser is determined by its type, Gas lasers tend to be based on the electrons in an atom being excited to higher energy states and then releasing that energy when the wave is reflected back into the cavity to cause stimulated emission, vapor, ion, and chemical laser work much the same way. Color depends on the electron valence bands. In a diode laser the color depends on the band gap in the materials that form the PN junction where the length of an electron's travel from one portion of the material to the other determines color. Much like a gas laser the electrons can be forced to move with reflected light reintroduced into the gain medium and cause stimulated emission. Some lasers excite molecules optically such as dye lasers which split up electron pairs, they split and can be forced back together with the light being reintroduced into the resonator thus causing stimulation. DPSS lasers operate on crystals that have dopants, usually rare earth metal ions with electrons that can be excited and they will drop to the ground state releasing photons, more information can be found by looking up 3 and 4 level gain mediums. Then you have sum frequency generation (SFG ) which mixes 2 wavelengths ( such as yellow DPSS ) If 2 identical waves are mixed it's called second harmonic generation, and depending on the fundamentals you can get third harmonic such as 1064 and 532 in SFG performing a third harmonic generation (THG) of 1064 resulting in 355nm. Power scales with the amount of input energy vs gain medium/stimulation.