Well, is there a particular demon design you're thinking of? If not, I recommend looking at different demons from mythology to get inspiration. It's what I would do. Oddly enough, right now I'm trying to find the color scheme for a character I just made! I'm thinking about looking at lightbulbs...
Oooh snap! I see that you're a person of culture as well!
As for Harmoknight and Hyper Light Drifter, Harmoknight is a kinda old 3Ds game released back at around late 2012 - early 2013. Its a rhythm game and is super cute. Its directed by one of my favorite artists ever! Still have the game on...
Harmoknight, Hyper Light Drifter, the LISA trilogy.
Dang, would I like to do a Harmoknight RP... But hardly anyone mentions that game anymore. Last time I heard of it was when people were discussing about how GameFreak wanted to make new games that weren't Pokémon, and they looked at their past...