It might dry lighter, so I'd wait till it's fully dried, but I had to paint grout once and it's not that difficult, it also acts as a sealer, so don't seal you grout, it will prevent the paint from adhering well. Do a small sample and wait for it to dry, it dries lighter. Another option is to mix white grout paint with water and "white wash" the grout until you get the color you want. Whatever you do, wait till the grout has had time to cure.

I always recommend doing a sample board up with you choice of grout before you commit. Grout isn't that expensive and sometimes the coloring samples aren't very accurate. I just hot glue some tile to cardboard and mixed a little water with the grout and it gave me a much better idea, what the finished product would look like.

On another thread, someone mentioned grout colorant by Aquamix, so I guess you could use that. But yes, I'd wait for it to dry, because it will dry lighter.

Can you post a photo?

I used Delorean Gray in our kitchen with white tile in 1986, and at first thought it was too dark also.
Give it time, it will lighten up and be fine!

I had someone do a beautiful job tiling my bathroom floor. They wanted me to get my grout and some other things. I went to home depot and got white grout. My tile with retro white with black. Now the pattern to the tile has disappeared. I am sick. It was my fault. I don't know what to do. I had been debating about a light grey and obviously wish that was the box I had picked up. I'd just like to get a little color on the grout. It has not been sealed yet.