Hello and thank you for your question.

Yes, the color of grout can be affected by the amount of water used. Often the grout will dry to a lighter color than expected/desired if too much water was used, though the opposite is also possible with some grouts. This is especially common in types of grout that contain polymers, as the chemical reaction between water and polymers can cause unpredicted color/shade changes.
There are sealer/color enhancer products that can be used on existing grout to try to correct color/shade issues. Of course you should try these on a small, inconspicuous area first and allow to dry thoroughly to see if the result is acceptable before using on a large area.

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Thank you.