Most harmonicas do have the key stamped on them somewhere, Jack. Colours are not good because they come off in your mouth. Grabbing the right one can be problematic in a darkened pub, admittedly. Another issue is that the harassed harmonica player often picks up the harp the wrong way round, thereby uttering an inappropriate ugly squeal instead of soothing harmonious lyricism. And I've deliberately left myself open there for your next quip... ;-)

I line mine up in front of me so that I know what's what. At least the bigger tremolos and chroms do look different to the little jobbies.