Is is easy to have areas of greater dye penetration with stain? Does it usually look good or not? I am thinking of staining light grey a shade darker grey.
Is is easy to have areas of greater dye penetration with stain? Yes and no.
Previously you wanted to know if is was possible to color the grout without removing it. The general answer is grout stain. I said that use of a stain might cause splotchy results. That was because you stated that the grout was fresh, plus it is your intention to move the color an infinitesimal degree within the same hue. Neither one of those conditions are prime conditions for uniform results.
One of the ingredients in many grouts is White Portland Cement. Portland cement does not cure in a day. It cures over a period of years. It can reach serviceable conditions in minutes or hours (with additives) but curing to serviceable conditions generally encompasses a range of a week to a month. (Blurb: apply silicone grout sealant after 7 to 14 days). 7 and 14 days are two accepted cure periods for serviceable concrete (portland cement hydration). (Blurb: apply grout stain to cured grout only). After 30 days the moisture content of the grout (including the Portland portion) is more or less stable or at least as stable as it's going to be for a very long time to come.
Given two samples of wood. One is green at 19% moisture content and the other dried to 12%. Wood stain is applied uniformly to each one. Which of the samples should be expected to produce splotchy results and ultimately fail? Regardless of the substrate, expect splotchy results when the moisture content is too high or when it's unstable.
Portland cement has a high initial Ph. After 30 days the Ph tends to descend and normalize. Alkalinity can contribute (via chemical reaction) to splotchy results.
Next, is the hue condition. Moving up or down small degrees within the same hue is difficult at best. For uniform results the old color must be completely blocked and the new hue cannot exhibit transparency or translucency. The new hue must be opaque.
Given an absorptive substrate with an unstable moisture content and a high Ph, will results of a single or multiple applications be uniform? I would expect splotchy results.
Does grout stain usually look good or not? When it's applied according to the directions, and when conditions approach their ideal state, stained grout meets, and sometimes exceeded, the appearance of the original grout.