Lighter than expected grout is a very common problem. Do you have any chunks of excess grout (in a bucket or in the garbage)? I ask because it can help you determine if my advice below may work for you. If you have a chunk of grout, crack it in half and see if the color inside matches what you originally intended. If it does...
...then your true grout color is hiding below a very thin layer of light grout. You either have a bit of efflorescence, or you washed a bit too much of the pigment off the top layer of grout. Either situation is correctable w/o too much work. After the grout has had about a week to cure, you are going to remove a very thin layer of the grout to expose its true color using a mild acid solution. You need to test this in a small area before committing to the whole project. Use a 50:50 mixture of white vinegar and water. Wet the grout and let it dwell for a minute or two. Then use a scrubby pad or nylon bristle brush on the grout. Then rinse it real good with water a few times. If you have a wet vac, it helps out during clean up. Anyways, allow this test area to completely dry and examine the results. If necessary, you can up the vinegar to full strength to get more results out of it...but it pays to start with a weaker solution to see if it works.
Tonto Goldstein….. but my friends call me Bubba
"The best way to get a bad law repealed is to enforce it strictly"
- Guy on the five