Just before I accidentally switched off my ipad I had commented that I didn't think Acanthus needed a companion. It's usually grown at the back of a mixed border/bed (to give height) or on its own as a specimen plant.

If it's in a border/bed I would probably go with soft colours such as cream, pale pinks or lavenders. And I'd choose something light and wafty to contrast with the solid architectural acanthus. Aquilegia, verbena bonariensis and thilactrum (the ones that look like an exploded firework/ spider with too many legs) come to mind. Homebase have a nice cream thilactrum which was shown at Chelsea.

Floaty grasses like Stipa gigantea or S. tenuissima; if I chose a thalictrum it would be delavayi so the purples would play off each other.
I might try Achillea 'Cloth of Gold', for fine textured foliage with yellow flowers on a strong horizontal line, against the coarse texture and vertical accents of the acanthus.
Mostly what goes with acanthus in my garden is glyphosate, but then most people say I'm odd...

Whatever you plant near it will soon be surrounded by it - it sends out surprisingly long runners. Took me three or four years to get rid of all of it. Pity, the leaves are so attractive. I'd go for annuals or biennials to give you a chance of clearing the ground in between.