Light frequency is a bit more objective than color and includes things we can’t see (like ultraviolet). So you can create any color by starting with a few distinct colors and then moving your light source either toward or away from an observer to Doppler shift one of your colors to the target color. So, say you want to create a very specific color of light with as little “waste light” as possible. It is possible to create laser light at any frequency between microwaves and X-rays, but the technique is a long way from efficient. If you want to efficiently create any very specific color of light, you just need to strap a laser to a starship.