Hi - unless you are about to enter fourth year I really wouldn't worry about this - chances are the wellies you'll be using on all your pre-clin placements are not going to survive to rotations which don't begin until halfway through fourth year.
I think they also want steel toe capped ones for the first few years (wouldn't bother with that requirement myself) and the ones they link to are black.
Brown will be absolutely fine so long as they are proper actual wellies that won't fall apart and have decent grip