You might look at euonymus fortunii - there are several variegated varieties, both gold and silver. They are happy north facing and stay bright and cheerful all year round.
If they should eventually get too big they are easily clipped and the clipping root easily so wil have little ones to sell at coffee mornings etc! My favourite for a dark corner is Emerald 'n Gold.

Lots of perennial geraniums will flower happily in that situation too, so you could try one of the tougher ones - Spritshenry is the one to ask about the best ones. (a blue one would look nice with the Emerald'n Gold while the latter is growing big eneough to fill the tub)