I used GroutShield as noted above, now about 2 years ago. We matched the existing color, but i do not see any reason why this product would not work very well for changing the color. I would give GroutShield a call, and ask them directly. Also, I found it much easier to use a shower squeegee instead of the provided sponge to remove excess product from the tile.
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Pennsylvania, for 60+ years, most recently, Allentown, now TV. 
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