That's a great question. I'd love to see what you find out too.

See if something like this is what you're looking for: http://www.behr.com/dsm-ext/v/index.jsp?vgnextoid=91b4f92a2d0cb110VgnVCM1000006b0910acRCRD#vgnextoid=91b4f92a2d0cb110VgnVCM1000006b0910acRCRD;view=17;channel=PROJECT_CENTER

It says it's for indoor/outdoor use so it just might work, depending on what kind of finish was applied.

Concrete Network is one site you may want to look up, if no one on houzz has experience with this sort of problem. The Concrete Network people may have a solution. Good luck! Let us know if you find an answer. I'm curious myself.

There's a process called acid staining that may work for you. Basically you clean and prep the area, apply the acidic stain that penetrates the sealer that's there. Then you neutralize the acid and clean. Then seal. There are tutorials online and I'm sure there's a professional in the area that could do it for you. It won't completely change the color, but you could get a wash of charcoal with the brown showing through, which I think would be interesting. Good luck!

Thanks,
I will continue to look into it.