Mmmmm, beige is not too bad. Hides baby spew reasonably well.
Black bleaches out too quickly and shows up everything.
Mine is red, it's unisex, but doesn't necessaryli go with all your cloothes.
Cause that is probably your main consideration: as the wrap will almost become part of your outfit, it should go with most of your clothes.
If you can find it, a slightly patterned fabric would be great at hiding stains.
Grey tones would be pretty good at going with most colours.

You should post a piccie, when you're done.

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