Age of the Grout
The age of the grout within the grout joint is one of the leading factors to a grout color that begins to fade to gray. Just like any other substance, grout and its associated colors are subject to time as well as wear and tear. While it can take years for grout to begin to lose its color if proper installation methods were followed, it will still fade over time and leave you with gray joints as the grout reverses back to its original concrete base.
Improper Mixing
Improper mixing during the grouting of the tiles can lead to the grout turning gray within the first few days or weeks after an installation is finished. Dry-mixed grout has a variety of pigments as well as polymers within the mixture, and for those ingredients to mix completely the proper mixing procedures must be followed. Always follow the manufacturer's instructions to the letter when it comes to mixing your grout, otherwise you run the risk of the pigments not mixing completely, leaving your grout discolored and gray.
Bad Batch of Grout
While it is rare, a manufacturer can on occasion sell a bad batch of grout. Also, if grout is left on the shelf for a prolonged period of time, the pigments within can settle, or in worst cases, be affected by exterior humidity that can cause them to chunk up within the bag. If bad grout is mixed and used (with the offending chunks removed), it can lead to the grout missing some of the key pigments that add color to the grout. If you open a bag of dry grout and there are clumps within, you should replace it with a fresh bag of nonclumped grout.
Excess Water During Grouting
The washing phase of the grouting process is just as important as the mixing of the grout itself. If too much water is used during the wiping up and cleaning up phase of grouting, the extra water will dilute the concrete-to-colored-pigment ratio. Once the grout begins to dry, it will have more concrete than color, which means gray will be the prominent color. Always wash the grout with the least amount of water possible, according to the manufacturer's instructions, to ensure proper color.