You could try traditional stairwell plants such as:

Chinese Evergreen (aglaemia)
Maidenhair Fern
Italian Bellflower
Kalanchoe
Begonia Rex
Sweetheart Vine
Spiderplant

More adventurous would be Fatsia japonia or Zamioculcas.

If the pots are to go outside a Euonymus Fortunii looks good in a shade pot and takes hardly any maintenance. There are silver and gold variegated varieties and they are all evergreen. If they get too big eventually they are happy to be trimmed.
A Christmas box would be nice too - another evergreen shade lover that has wonderfully scented small white flowers in late winter .
Spring bulbs would be OK (probably not tulips though) and for summer colour you can't beat little begonia sempervirens, which you can buy as summer bedders. They are in shades of pink and red and flower all summer whatever the weather in sun or shade.

Re-reading this - outside flat does not mean inside a close! ignore most of my suggestions apart from the Fatsia as they will not take a frost. Mahonia, Japanese Anemone, Hellebore will all do well outside in pots.

Just an afterthought - the euonymus may lose some of the brightness of variegation in very dense shade - try one and see. Emerald 'n Gold is a nice bright one and very good tempered. (Easy to take cuttings from too)

Hardy cyclamen (C. coum and C. hederifolium) could go in smaller pots at the front. They flower in autumn, winter and spring and have beautiful marbled leaves after the flowers have finished. I don't understand whether the pots are to go outside or inside.