If going for foliage rather than flowers, herucheras have a wonderful range of dark reds, chocolates to purples. If dead headed, some have flowers that will come back through the season. Good for the base of planting arrangement.
I have found that there can be problems with flowers that are so dark red that you can't really see them. Perhaps the silver will mitigate this.
There are some lovely black cherry / black red sweet williams which are pretty tough plants.
I agree @JennyJ - having seen a whole thread on Red Velour here on the forum I bought some and they are gorgeous - but too red for our purposes. Hmm @Fire - those dark dianthus are really nice! Getting lots of good ideas here folks - thank you!
There's a petunia called black velvet which is a very dark burgundy.
Euphorbia purpurea has quite a dark burgundy leaf and is one of the smaller ones - I've got some that have been in a mixed pot for two or three years and seem to be pretty well behaved
Gardening on the edge of Exmoor, in Devon
“It's still magic even if you know how it's done.”
Thanks @Fire but we have a local nursery that grows the plants for us and I'm not sure that poppies would work well in planters and hanging baskets and keep flowering all season.
You did mention troughs. It might be interesting, and cheap, to put in some annual seeds at different points and see what happens. A trough with 'wild flowers' might be quite stunning.
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Hmm @Fire - those dark dianthus are really nice!
Getting lots of good ideas here folks - thank you!
Euphorbia purpurea has quite a dark burgundy leaf and is one of the smaller ones - I've got some that have been in a mixed pot for two or three years and seem to be pretty well behaved
“It's still magic even if you know how it's done.”
https://www.panamseed.com/plant_info.aspx?phid=048602869030939
Note it is Easy Wave not Tidal Wave
https://homeguides.sfgate.com/different-kinds-wave-petunias-62912.html