I'd go for some Ivy as a permanent year round backdrop first
It'll completely cover the wall and provide good habbitat for birds and bugs
Then play about with plants for colour from there.
Or, you could use the opportunity of a big blank canvas and go all creative on it maybe
Colour or big panels of garden art would look quite funky on it I think, or you could put in a cascading water run from the top of the wall down (with a return tank hidden somewhere) and cover the wall with mosaic designs. It all depends whether you want to make a feature of it or not ?
I'd be tempted to get all arty (but that's just me
)
Good luck and show us photo's when you're done