My pots are not as impressive as Harlow Carr's!
I tried to do them in blues this year.
Successful were the lobelia both dark and light, much underrated plants I think. Also a success though a bit thin and weedy at first, were the annual nemophila - baby blue eyes - the flowers are of such a stunning, almost turquoise blue that they are worth having. Succesful too were the delphiniums but they are fragile and do need staking. I painted the canes with blue paint, but it washed off in the rain!
A failure - the 'Sky Blue' petunias. Firstly they are not sky blue but distinctly lilac and they don't stand up to any rain at all, but worse, they die horribly, leaving a sticky grey mess. I won't use them again.
However I'll consider it a trial run for next year, when I will use all but the petunias again, looking out for better performing ones or using lime green ones for a light touch - all blue can be a bit sombre.
I'll start the nemophila menzii early in the greenhouse, and will add some dwarf cornflowers, find a better paint for the canes for the delphiniums, and blue string to tie them up with....
Gardeners are nothing if not optimists, don't you think?