Specter, since he can't die he is sure to be the killer. and freaky ghosts like him can control every kind of stuff, he probably made the trap for someone, that was the Game Tester, AKA Tangerine
I'd guess Maroon, since she is a Chef, she could put that poisonous gas inside the cooker in a way that no one would notice, it couls be the same gas that made the tiger attack Ivory
Btw I think that Pink is the killer, since she STILL dislikes dark colors, and Teal is a a dark color, she could've used the pipes to go to the Library, then read the book. then to kill Teal with it...
Cobalt: *shocked* Grr... TEAL! why did you have to go so early... I promise you, i'll avenge you... (after i make this genderswap potion of course)