*Hela* is not limited to colors - but you seem to have a good grasp on the basic differences. You can have a *luma tago* and a *hela tago*. *Luma* means there's a lot of light. *Hela* means it's "bright" (which means a lot of light) or that it's "light" in the sense of having white mixed in - like light grey.

Check out the definitions in PIV. *Hela* is defined essentially the same as *luma* with a few secondary definitions added on.

luma

 1. Daŭre lumanta

hela

 1. Donanta aŭ ricevanta fortan vivan lumon
 2. Enhavanta multe da blanko (hela ruĝo)
 3. Vigle, akute sonora (hele sonantaj trumpetoj)