My guess is BLUE.

Unfooruman's cloak is a deep blue, am I right? It's like Fooruman's purple robe, but a darker and opposite shade of it...which would be blue. So it'd make sense to associate Unfooruman with blue.

Red could possibly be some kind of hell and firey brimstone target, so we can't really risk it.
Green may be back out to the island, where we would have to return back to the citidel all over again so we can't risk that. Yellow, I'm thinking electircal. Though blue could be the freezer, so we can't be completely sure.

But I like ice, Ice is my natural element, so I'm going with BLUE.