Many of the Warlock pets have Glyphs that give em a Fel like appearence, The only pet that does not have a fel like apperence is the Voidwalker.
Does it bother you just a bit that your Warlock is fully covered in Green and your Tanking pet is Purple? Well sometimes yes.
So Blizzard should give Warlocks a Glyph that changes their Voidwalkers appearence into a Fel Elemental. the pet could then be called, “Felwalker” so that people know it’s a Voidwalker Glyph