I had the same issue and decided to try to brighten things up this winter. I second heathers and cyclamen.
I'm trying some heathers for their foliage (e.g. Calluna Vulgaris Firefly) and some for the flowers (e.g. Erica Carnea Challenger - which ok is technically a heath).
With the cyclamen, if you get some Hederifolium and some Coum, you should get quite a long season I think. At least, that's my plan.
Another thing that I'm trying is a winter honeysuckle such as Lonicera × purpusii 'Winter Beauty', which I'm trying up against the railing of my balcony.