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Unfortunately, grout companies have a disclaimer on their products that says: Grout colors shown here may not accurately represent the actual color of the installed grout because of job-site conditions, product preparation, installation methods, lighting, and the type of tile or stone utilized. They do this to protect themselves in situations like your own.
Unfortunately, they are correct about alot of it; often the type of water can influence the outcome, as well as, using too much water to clean the tiles; grout should firm up and be cleaned as dry as possible. Grout needs to be mixed as close as possible to instructions.
If the color is far off, and you think you have done everything correctly, you can contact the company and maybe they will compensate you in some way. Another option is to purchase grout stain/ paint that can be applied to the grout to get the desired color.
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