*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.