That wall is gorgeous, and crying out for an espaliered fruit tree of some description! Choose one carefully and you could have blossom, fruit and autumn leaf colour.
Oh sorry, you were asking about plants for the border.
The hardest season for all round colour is going to be winter, you could try a cornus for a real splash of colour, one decent one would do, it would warm your heart every time you saw it. Underplant with bulbs for spring.
You have a seating area there so I would want something scented, lemon balm perhaps which is a nice colour too. Summer is easy really, good herbaceous plants that look after themselves cranesbill, salvias and penstemon. Maybe some crocosmia, doesn’t come more colourful than a lucifer. I would also like a bit of height and movement so would put a pretty grass in there somewhere.
Some sedums for autumn colour, and bee friendly too.
But, oh, that wall.............