Perennial geraniums apparently.
I have an east facing garden with a North facing bed which is an absolute ballache as I love colour.
Fortunately my best mate is a gardener and she's recommended the geraniums and foxgloves (if you don't have outdoor pets/small kids - they're very poisonous). Also Mahonia and hostas which are a large leafed shrub rather than flowers but come in some amazing colours and patterns. You will need to slug proof though as they love hostas.
If you have the money you can buy a ready planned 'shady border' plant kit here from Homebase for £150.
Or a smaller scale, cheaper option from Jersey plants direct.
I'm also planning to cheat by planting some more colourful, less shade friendly plants in tubs and move them around so they get the sun, but plonk them back in the border if I want a bit of colour on that side of the garden when I have people round.