You need an epoxy grout stain. It seals and colors together.

Thanks! Looks like I'll need to remove the old grout for that, which is too much work and maybe too delicate (due to some very thin grout lines).

I think I'll spot fill the tiny pockets (in grout lines) with caulk, and then seal the whole thing up.

NOOOOOOO! Don't use caulk! You will make an uncleanable mess. An epoxy grout stain goes on just like a sealer, with a little bottle. It just has color in it that makes the grout look fresh, and then seals it. That's all you need.

Oh... I see. I was too quick in my search and thought it applies on like grout. Great. I'll go search for them. Thanks so much!

I am wondering about the same thing. We have 1" original (1928) mosaics in our bathroom. We need to re-set some tiles that are loose, and dh was unsure what to do with it. We can put the tiles in using grout that is 'close enough' but then surely it won't match.

If I use the grout stain, we would have to do it line by line, even with the applicator brush seems tedious. If we regrout the whole thing, that means 'sanding down' the whole floor. Also tedious!

Since you already have it, my vote goes to the Fusion Pro. Just use a utility knife to score out the old stuff.