A box (Buxus sempervirens) ball or cone either side would be quite architectural, but would need to be clipped regularly to keep the shape. Otherwise, lavender, though not sure any grey leaved plant will contrast that well with the colour of the wall behind. The smaller phormiums (such as P. Cream Delight or for reddish leaves P. 'Sundowner') would do there, and if you don't mind deciduous plants, Berberis thunbergii atropurpurea 'nana', or Berberis 'Bagatelle', which forms a globe of red foliage in spring, summer and autumn. None of these get that large, all will do well in full sun - you really need something with a height and spread of no more than 2-4 feet. Cistus varieties such as C. corbariensis (the hardiest) and if you don't live in too cold a region, C. 'Peggy Sammons', both evergreen. Cistus, though, are best planted in Spring.

Thank you for the advise I should have said I live in the Central belt of Scotland, will this make a difference to the recommended plants?

How about Photinia fraseri nana? (Not to be confused with other photinias which get a lot bigger - this one stays small)

Or instead of shrubs you could try planting a couple of climbing roses trained along the walls and have colour all summer and early autumn. As long as they receive more than 6 hours of direct sunlight they will do well. If the walls aren't long enough just plant a couple of bush roses, eg Just Joey or A Whiter Shade Of Pale would look great.

I wouldn't risk the Phormiums then, given where you are, but the others will be fine, apart from possibly certain varieties of Cistus. Another one to consider, if your soil is not alkaline, is Pieris, but make sure you choose a Pieris japonica variety and not P. forrestii (these prefer shade) - P. 'Little Heath' is an attractive one, and stays small.