Among annuals: green-leaved forms of Wax Begonia, Star Cluster (Pentas lanceolata), many Primulas, Hybrid Monkeyflower (Mimulus hybridus), Wishbone Flower (Torenia fournieri).

I have some pots which do not receive a lot of sun which are planted with heucheras. The leaves are different colours, so you get a much longer season of interest than with flowers. If you wanted something with a bit of height, you could plant a grass or a small phormium in the centre of each pot as well

Given that you live as far north as Bulba and I do forget Tugbrethil's suggestions! Are you looking for annuals or perennials? Andrew's suggestion of heucheras is good if you like them (we don't) and I'm not sure a phormium will survive. You could get a lot of spring colour with small bulbs such as crocus, iris, minature daffs and iris. Summer could be bedding plants from your local GC and a couple of colourful shrubs to all round colour. Question is how big are your pots and will you have to keep them watered?

Not enough heat for any of mine?? Drat! I would have thought that Primula polyantha and the Monkeyflower would have worked, at least, since we do those in shady areas in the middle of winter, here. Ah, well...I guess that it's a new meaning for "Culture Shock". : )