I have a similar but slightly different request.
See, here are two different workspaces in the same instance of Opera, see the differences???
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Me neither! You can't even be sure i took screen caps of two different workspaces but I absolutely did, there is no possible distinction and without going to the autohidden side panel no way of knowing.
But what if the hue was a little different?
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Ok, I just rotated the hue for the whole image so the Opera logo was affected, but that subtle change makes it easy to distinguish instances.