What I use is basicly a "slab" of krill. By that I mean a frozen sheet of frozen krill that I brake off a little bit and feed to my eel. I sparingly feed him live, though, because it worries me that he may not go back to frozen.
When my eel is hungry he leaves his little "cave" area and will start swimming all over the tank. I've never noticed any color changes, but then again, my Eel isn't that old.
And I noticed you have Bumblebee Gobies and the eel in the same tank. The eel doesn't eat them or anything? I was really thinking about getting 2-3 of them, but in the end decided I didn't want to see the eel gulp them down in 5 seconds.
Edit: I actually just noticed this, but my Eel is a little lighter in color (he hasn't eaten in a bit). So yeah, that's probably the reason.