shadowd15 submitted a new mod: Slime Armor Colors Mod - Makes the color index actually work. Read more about this mod...
Er... Well, if everyone installs it it works. Since it's modifying the vanilla object, in theory in multiplayer it'd just show up as green to anyone who didn't have the mod? I've never really understood "multiplayer compatible". All mods are if everyone installs them.
/admin /spawnitem slimehead 1 '{ "colorIndex" : # }' /admin "slimehead" can be replaced with "slimechest" or "slimelegs" (I think it's slimelegs, but if that doesn't work it's slimepants) and the "#" needs to be replaced with a number 0-13 (I think, just try it out. Eventually the colors start cycling.) Hope this helps!