
I never have a problem with my slings Cara, but always get confused with yours!! But a colour system for slings isn't a bad idea for my next set...
Anyway, its all to do with who makes them and causing maximum confusion and choas on the crag thus leading you to buy the same manufacturer for all you gear hence the colour codes work. Cynical, me never!
Personally I actually don't know what colour any of my DMM or HB nuts are, I just grab the karabiner and stick in the one that looks like it will fit. I think after time you get used to which nuts correspond to each other.
To me anodising the nuts themselves in different colours seems pointless as after a few uses they'll be scratched to hell and back anyway. In the same way, my hexes no longer have their sizes printed on them after use.
I think Needlesports detail which nuts match up to which in their product descriptions.