Grout With Sealer And Xit What Is The Best Grout Sealer?

What is the best grout sealer? - grout with sealer and xit

We had a white tile 4 x 4 inches to the bathroom and looks beautiful. I fear that the milk of overtime might be dirty. I would like to apply a grout sealer, but I do not want to know the color of the grout to change. Darken the application of grout sealer? What is the best sealant for this purpose? Thanks in advance!