Sheep: Black skin [Done, added dark brown too]
Creeper: Lighter and darker green [Done]
Spider: Brown [Done]
Cave Spider: Gray [Done]
Enderman: Green Eyes (maybe)
Phantom: Green Eyes (maybe)
Endermite: Green eyes (maybe)
Ender dragon: Green eyes (even more maybe than the other three)
Blaze: blue (perhaps for soul sand valley biomes?) [Done]
Squid: various colors [Done]
Glow Squid: various colors [Done]
Dolphin: Different variants based on real dolphins [Done, variants based on common and striped dolphins]
Slime: various colors (Rainbow collection of slimes would be sooooo cool) [Done, and yes, it is very cool]
Guardian: Shades of blue [Done, added green variant too]
Elder Guardian: [Done, added green variant to match green guardians]
Zombie Variants: Clothes to match Alex skin