I have planted Carex ‘Evergold’ in one of the small, square beds set into the gravel and I think flowers would be interesting standing up above the grass. This is the ‘hot colours’ part of the garden and so I think quite a dramatic combination could work well: Astrantia, Sanguisorba, or a purple Sedum.
Astrantia (www.crocus.co.uk)
Sanguisorba ‘Tanna’ (www.crocus.co.uk)
Sedum ‘Purple Emperor’ (www.telegraph.co.uk)