Any of the mainstream penetrating sealers are fine. Paint it on the grout joints and watch it dissappear into the grout. Let it dry and do it again. Don't get it on the tile and when you do wipe it off immediately. Don't wait until tommorrow to clean it up.
You could paint it but.....use a bottle with a nozzle. Hair dye bottle or similar. Cut the nozzle a bit smaller than the grout joint as the liquid will spread. Use a paper towel to wipe up and overage. It's really the best.
Don't let the sealer puddle in the joints. You can wipe that with paper towel as well. I never let the sealer dwell on the tile or joint as long as the instructions say. I'd rather do it twice than have to scrub too much away.