Purple foliage plants for shade?
I am stumbling my way through trying to redesign my garden, and I was hoping to ask for some advice from some of the knowledgeable people on the forum. I am currently replanting a small, shady flowerbed in my small, shady London garden; it's against a brick wall, with a small apple tree in the middle of it. The soil is a bit clayey but actually not bad; and the bed gets some sun at the front but it's mostly in shade at the back so all the plants need to be shade tolerant. I have gone for a colour-scheme of yellowy-green, with some notes of purple.
My question is about the back of the flowerbed - I have two clusters of three euphorbia amygladoides robbiae at the back, acting as a greeny-yellow backdrop to some slightly shorter plants in purples and yellows eg geranium clerkei 'Kashmir Purple', and a cluster of corydalis lutea. The clusters of euphorbias are at the two back corners, but there is a gap about 80 cm across left between them, directly behind the apple tree. I wanted to fill in the gap with something that would provide some structure behind the tree without being too eyecatching (as it will be partially hidden), and possibly provide some contrast in keeping with the colour theme.
I was wondering whether it would be a horrible mistake to put in euphorbia amygladoides purpurea - the same shaped plant but in a contrasting purple/bronze? But I have a feeling that that may be overpowering, and make it look as if the bed is striped like a mint humbug? Any ideas as to what would work in the space? I am a complete novice at this and most of what I have learned in the last three years has come from getting it wrong the first time round, so I would really appreciate any advice to help me shortcut any design disasters!