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.