Hmmm.
I suggest googling 'plum curtains' and clicking on the 'Images' option. You'll then see lots of pictures of plum-coloured curtains, set against various backgrounds.
I've just done that and I'm drawn towards a sort of pinkish-grey colour, such as 'Pink Bunting' here:
http://www.dulux.co.uk/colours/ready_mixed/pinks_and_violets.jsp
(Hover your mouse over the colours to see the names. Click to see a colour in a room setting, together with a complementary colour which goes well with it).