Welcome to the forum, Johnson.
I wouldn't be too quick on painting that grout. Grout drying lighter than expected is a very, very common complaint. And a lot of pros who practically ruin their wrists twisting their sponges as dry as possible are given the standard industry response to light grout of, "you used too much water". While it may be true in your case....maybe it's not.
I'd wash it with vinegar (or escalate to something stronger like sulfamic acid), let it dwell for a minute or so, then scrub it with a nylon bristle brush a bit and rinse it off (use a wet vac if you have one). Rinse it a second time with water to neutralize the acidity and allow it to dry. Inspect to see if you have an improvement.
Time for some patience...
You only grouted two days ago
and it's not cured enough to be attacking it with an acid like this. So wait 'til this weekend before performing a test in an small, inconspicuous area. If you have your results you're looking for, proceed with the remainder of the project.