It's possible someone painted the tiles. Glazing doesn't usually wear off.

Has she tried testing with paint remover or a heat gun to see of it's beige paint?

My craftsman tile was painted white. Hooray for real estate agents.

Thanks! Both the blue and the beige appear to be glaze, not paint--or at least there's no textural difference to indicate it might be paint. If anything it looks like the blue would have been on top with the beige underneath. I was thinking It's more like when you have old white bathroom tiles (like I do in my house) and the top layer starts to wear off, so they look a little gray. But I don't know how that would happen on a vertical surface.

dirt_cred, sorry about your white painted tiles!

http://www.pewabic.org/wp-content/uploads/2014/07/Glaze-Palette_2013_web1.pdf

It's quite likely that the orignal glazes were applied to create the effect you see. The page I've attached from Pewabic's site shows some sample of what you describe. When the tiles were orignally created, much of the work was done by hand....thus the variations in color.