Steph - as you know I'm a 3 often (i'm a 3-3.5 usually), but I went for a 4 and they are still snug - I am wearing my black ones at the moment - I have worn them in the rain and my feet didn't seem to get wet. Depends if you will wear socks with them too - I do as I haven't yet plucked up the courage to wash the lining! But I have got army green off ebay - I know it's not a proper green, but if you really want green they are nice!
Best advice would be either find somewhere to try them on or use a good website where you can send back for an alternative size if need be. I guess it depends how big your usual size caymans are - if they are sometimes just fitting rather than loose, I'd go up a size!
Good luck - they are great - I have choc ones too - my feet are so toasty - I couldn't possibly just wear them indoors!
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