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I would say it's a lighter purple than dark, but it's got that two-tone look about it and that's why I wondered about black. I think I might be inclined to go for a flesh/ivory coloured bra, having thought about it, especially as the dress isn't designed to be a tight fit round the bust area - in other words, boobies will be on display rather than hidden!
I buy my underwear from Bravissimo but they don't have anything different to what I already have. I had a quick look at figleaves before posting my Q but I'll have another go and see if anything catches my eye.
Thanks for your help, you two.