Possibly efflorescence. Scrub with an acidic tile cleaner designed to remove it. The other possibility is exactly the opposite, which is that someone has used acid to scrub the bath too many times and stripped the color out of the grout. It’s hard to know what’s the cause without a closer inspection. But, you can’t do much worse damage if it’s already acid damaged, so Id try the efflorescence remover first, and then assess. Good luck!

Yep. Do all that, and if the result is the same, there are grout "colerents" that you can apply. Aside that, the only other recourse is to dig it out and re-grout.

I'll give it a try. There is no ventilation in that room. Are there efflorescence cleaners that I can use in spaces with no exhaust fans? If not, would vinegar and water work?