Try a Clorox pen or paint them.
Lynn. Lowes and other hardware stores sale small bottles of color to recolor grout. I did mine in kitchen a few years ago with damp rag and toothbrush. Took a little while but worked well for me.
Lysol Lime & Rust Remover Toilet Bowl cleaner, comes in a black squirt bottle. Scrub with Scrub Daddy or Mr. Clean Magic Eraser. If stubborn, use a scouring stick to scrub.
If you can't get it clean, paint it.
Once you have it white again, seal it.
Thanks for the tips, I’ll see which one works best!
I've used Soft Scrub successfully. You could also use a baking soda paste.