it belongs in the chaos school of magic in wow and chaos magic is colored dark violet-ish black-ish... just like Chaos Wave and Touch of chaos (which both belong in the same spell school). The oddball here is Chaos Bolt which did chaos damage in the past, but was changed to shadow damage in Mists of Pandaria i believe...and yet its green.
Should just call Demon bolt Chaos bolt instead and give Chaos bolt a new name ^^.
The unique thing about this magic school is that its damage is chaos, meaning it combines all seven magic school and physical damage (if u get interupted while casting a chaos spell u get locked out of all magic schools at once) It also ignores some of the defenses that Physical damage ignored (like Ko'ragh shield, twilight ward in the past...etc).
tldr: it wouldnt fit lore wise to make it green, however looking at this picture they could make the fire in demon bolt green... now that would be cool!
(note: interesting thing about the animation is that it has Runes in it, might indicate some connection to Rune magic)