I've matched gray grout color on remodel by mixing various available grays from the grout line. Custom, in both cases. They have grays of sufficient variation that I had no trouble making what I needed. Lotsa little-bitty Dixie cups, big Dixie cups, popsicle sticks, and a heat gun. Takes a little time to get the mix right, but makes the customer smile to see the result.
A good impregnating sealer shouldn't be giving you any color change at all. An enhancing sealer should, but you'd wanna test that real well, too, including the 24-hour drying time. I'd try to get the grout color right instead, Gabe.
My opinion; worth price charged.