QUESTIONHow long should I wait to see if the grout installed on my kitchen ceramic tile floor is dry enough to determine if the color will become even?
The tile is ceramic penny round tile, and there's a lot of grouting. The original grout color is "platinum" which is a light gray. It was finished 16 days ago, and the weather has been humid. The colors of the grout now range from bright white to light and dark patches of gray. I haven't seen any increase in color uniformity since the installation.
ANSWERANSWER - Generally speaking, cement based mortars should be fully cured within 28 days assuming it isn't raining or is in cold temperatures.
I would expect that an interior installation of a cementitious grout should be cured within 14 days, if it hasn't been subjected to excessive moisture.
Even if the tile is a porcelain tile, which penny rounds normally are, and if you the tile is bonded to an impervious membrane, I would expect it to be relative dry and color fast.
Cementitious grouts are known for having what we call shade variation. The grout will tend to dry different colors for a number of reasons. If there was too much water used to mix the grout it can cause the color pigments to not be consistent. If too much water was used in washing the tile and grout after grouting it will pull color pigments out of the grout. If the tile edges have over glaze so the edges of the tile absorb at different rates it will cause the grout to dry at a different rate that can cause color variation. If there is a draft hitting only a portion of the grout, it causes the grout in those areas to dry faster and that might cause a change in color.
If you want to try to change the grout color, there are grout colorants that are epoxy paints that match grout colors that can be applied to apply to all of the grout to give it a uniform look.