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Any colour would work fine, I reckon. And yes, don't get a "flowergirl" dress. Get a girls dress, for way cheaper.
I agree that it's whatever you want. Mine will be in white with a sash to match the BM. It wouldn't hurt to talk to the FG and see if she has a picture in her head of what she'd like to wear. Depending on her age she may have a clear image of being in white (and thus be kinda bummed if she's not) or alternatively she may want to be in purple or something. You may not be able to give her exactly what she wants, depending on your colors and her choice, but it never hurts to ask