You can try darkening it up with an enhancing sealer. The problem is that if it doesn't darken sufficiently, you may have a problem using a colorant afterward, if you so desire. As for going over it with a darker grout, there's a good chance that most of it will wash out when you clean the excess grout off, and what doesn't wash out at that point will flake out once it dries.

In short, it's a bad idea.

Once you use the colorant, it's ALREADY permanently sealed. :-)

:) Oops, I guess I mean can I use a colorant/sealer now since they haven't sealed it yet? (and skip the enhancer/sealer--sounds like that won't be dark enough for me). I don't have pictures yet, but I know the wall grout is DeLorean Gray, and the floor is Mist.

Yes, you can, and regardless of what the color is now, or what you want it to be, it can be done, and relatively easily.

You said you've done some research and found the reference to Aquamix's colorants. They're the best there is. I have a link to where you can buy them on line, as well as a forum thread that gives explicit directions on using them.

Here's the url to buy them on line:

http://www.1877floorguy.com/grcopr.html

And below is the link to the forum thread. Refer to reply #2:

Here is a link that might be useful: Using Grout colorants

Thanks for the info, Bill. I was away for the weekend and didn't have handy Internet access. The good news is that my contractor is going to scrape out the light grout and replace it with the Charcoal gray grout I got today :)

Good deal. I love a happy ending! :-)