Toothpaste works well. Or just good ol bleach.

I have found that nail polish remover is the easiest thing to get dye out of tubs. 

Magic erasers work well and so does astringent (any kind with alcohol). Honestly, just using it regularly will get it out too! I’ve had my sink stained badly before and nothing would it get out immediately, but I noticed after just using it like normal and cleaning it periodically it came out eventually. So unless you need it out now, don’t panic because it should come out over time!

Hello! I dyed my hair blue for the first time myself today.  What’s the best way to remove the stains from my tub?? Thank you! 

Help! My tub looks like a smurf crime scene!