Oh I bet it was the bridesmaids!
I can answer the hair coloring question.
So when I do my roots, even though my color is dirty blonde, the dye itself looks dark. You mix it with a developer and put it on for one hour while it processes.
When the time is up, you jump in the shower to rinse it off. If your shower curtain is cloth, and you are careless, it could easily adhere in streaks.
Confess I have taken my hair coloring kit with me on short trips. Can’t be seen with unkempt roots when I want to be stylish on holiday. 
I’m always super super careful to cover the entire sink area where I do the application. Then I make sure everything that comes off is thoroughly rinsed away. There should be nothing left at all to make a stain on a towel.
It should never get on your pillow cases because in theory it’s all rinsed off by then.
I think it would be a little offputting to read a rule about hair dye on the shower curtain… Maybe just in your general house rules. Please don’t dye hair in our bathrooms. Thank you.