I'd love a dual, absolute/category, scale to make my decisioning easier. If something is green on an absolute scale then it's a smaller problem if I pick a product labeled red on on category scale. But I do know that I shouldn't do that.

Dual scale would make it possible to differentiate between two similar products - an "eco" cotton bag could be orange/green while a single plastic use bag could green/red. This way every one would know that an eco bag is worse than a plastic one unless you use it a lot.

Finally, producers would be motivated to optimise environmental impact of all kinds of products because having a red label would be shameful.