Just grouted my floor it is lighter than I considered necessary. I want it grey its almost white? ...
Answers: You have a few option here...some pretty easy to do and some not so confident.
Based on your description of the problem is sounds like you have a efflorescence problem. That's usually caused by getting too much river on your grout during the first clean-up step. The result is that the water pulls minerals out of the grout and coat the surface which get it appear whiter than it should be.
To remedy the situation you can get an bitter wash designed to remove the minerals. You can find it in juice or powder form but basically you a moment ago apply the solution and then scrub it next to a grout brush. You could also try vinegar to do the same entry. The problem with this solution is that it might be difficult to draw from an even color unless you can do the entire floor at once.
Another solution is just as wmwiv suggested and use some grout stain. Just apply it to the surface and wipe it down.
The finishing option is the not so unforced one. If you REALLY don't like the process it looks and the other solutions don't work, you can always remove the grout and do it again. You don't hold to take adjectives of it out but you want to get at smallest 1/4" so the new grout have enough to hold onto. It's pretty labor intensive since you hold to either use a Dremel tool or mitt cut it out. I've done this before and it's purely about even as far as which instrument to do it. The Dremel is less work but slower and you distinctly have to enjoy the tool. The grout saw worked well and solely cost a few dollars even with replacement blades.
If I be in your situation (and I enjoy been), I would try the other options first and use removal as a finishing resort.
Get some Grout Stain.