I think you'll find rainbow (and other fancy bright colours and designs) steths become old pretty fast, and then just look a little odd and out of place on the wards (I've heard some consultants even call them "unprofessional"). The vast majority of people stick with traditional colours like black, dark red, dark blue, etc. and I'd probably recommend a colour along those lines.
A steth is used for not only physical examination, but it also signifies your role as part of the "doctor team" (even if you aren't there yet!) in the mind of the patient and can instill confidence, I can't see a situation (other than maybe with little kids in paediatrics?) where having a fancy coloured steth offers anything but a potential disadvantage. I haven't seen this happen, but it would be terrible for a patient to think less of you because your physical examination equipment looked like it was straight from an episode of My Little Pony