Hi Fred, Welcome!
What were you using to attempt cleaning?
Can you post a photo?
Usually a tile & stone specific high alkaline cleaner will do the trick. It's very important to allow the product to dwell on the surface of extremely soiled surfaces for at least 10 minutes. It is essential to not allow the product not to dry on the surface, which would require reapplication.
Agitation with a stiff bristle brush will also promote premium results.
Removal of the dirty solution with a wet vac will facilitate cleanup, as well as minimize spreading of the soil.
Rinsing may or may not be necessary, depending on the products dilution.
I have used grout colorant on SpectaLock, with successful results. The key will be the epoxy will still need to be thoroughly cleaned beforehand. If you decide to go this route, I strongly suggest to perform a bond TEST before doing the entire area.