yes, I will be specifying the blue / brown convention for reliever/preventer, though for example the ventolin or generic equivalent is usually quite a dark blue whilst my Bricanyl (terbutaline) inhaler just has a paler blue around the base and the rest is white. (though at one time my mum had a salbutamol inhaler with a pale green product body .)
Based on this there is still some scope. I don't know whether (depending on what the thing ended up looking like) the customisation could be just a small detail such as a lanyard or the colour of the mouthpiece cover or a window to slide a picture in or something. Certainly it would not be total product body customisation as, particularly with the reliever, rapid identification of the inhaler is necessary for both the user, other people and eg medics in an emergency.
What I do need to do is look into the colours of the smaller-market inhalers, combination inhalers, etc. as I am not familiar with these. However I imagine I will still be working to the blue / brown convention.
Re your comments on spacers, I found this link on the design of new type spacers for kids, is quite interesting (and I want the caterpillar one!):
.) 