When you get to know the Eschers, by having a lot of labelled stones pass through your hands (with end labels identifying color) you could probably recognize an unlabeled stone, or know which color an Escher is that only has a back label.

Be that as it may, selling an Escher without at least a back label, even a partial one, is not going to bring near what a labelled stone would bring. IME the market ebbs and flows. They are going for half of what they were bringing a year or two ago. Speaking of Eschers. I don't know the other stones that well.