Tile – apply grout over grout to change the color

Don't re-grout, clean it! Talking with a Home depot guy last year about this and he was amazed I still had to use a brush. He shows me this stuff called oxygen bleach from Ajax. This was literally the easiest method I have ever used. Make a solution with just enough hot water to dissolve it, apply and let it sit for 15-20 min and mop off. That easy. I will never use a tile brush again. Once you get it sparkling white, you can upkeep it my adding a small amount every time you mop and you will have pearly white floors forever.

If you want to be sure, yes, you have to dig all the grout out first. Grout bonds mostly to the tiles, less so to itself. If you dig the grout out, try to do it at a 90 degree angle to the "proper height" grout, this will give you the best adhesion for your patch. Also, if possible try to make those 90 degree joins where there's more tile to adhere to (avoid starting a new grout line at the corners).

However 48 hours is right on the cusp of where I might try to fill it in. Typically grout takes a few weeks to fully cure, but is ready to seal after 2 or 3 days. That says to me that you might still get the new grout to bond to the old. I know there are cases where a homeowner might accidentally scratch the grout, in general, the tile mechanic doesn't always scrape up all the grout, before fixing something like that.

Keep in mind, if the new grout fails, it will be harder to dig everything out later than now. The grout is softer now, all the materials are at hand, and there's incentive to finish the job.