Chemical Symbols
I know I am stepping out of my preferred domain of Fizix and into Chemistry, but why not also include the internationally accepted chemical symbols O2 (subscripted 2) or N on each container instead of any words in English - different names for Oxygen in different languages. That way, any "trained " re-filler should be able to tell the difference immediately. Forget colour codes, because I certainly don't know the colour for oxygen when it comes in gas bottles.