Brad-
Tile-x is acidic-
Travertine is acid sensitive
Stone is compromised of minerals.
They don't lose their color
However because of the acid contacting the stone it will etch(mark of corrosion)
In the stone world an etch will always be lighter than the stone-
A true stain will always be darker.
Sealers, enhancers, colorants will not help you-
You need to fine a reputable stone refinisher who will refinish your stone and shower stall back to its original finish complete with caulking.
He or she will be able to recommend products for ongoing care .
A bit of information and stone education as well.