Both seem related to the light. Hel seems to be used for colors while Lum seems to be used for everything else. What is the real difference of meaning between them?
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.
- Daŭre lumanta
- Donanta aŭ ricevanta fortan vivan lumon
- Enhavanta multe da blanko (hela ruĝo)
- Vigle, akute sonora (hele sonantaj trumpetoj)