Weird world building that will always just be background stuff that is never actively brought up: Pink is the color of good health wishes in this world. Infirmary sheets are quickly washed of blood and returned to you with a pink stain if you’re going to remain alive long enough that they want to reuse sheets instead of staining a new set. So people will wear pink if they want to give themselves good luck in their own health (I drew a pregnant lady in pick when Dean was petting a dog) or give pink flowers to people who are wick or wounded.
Luckily yellows and whites that represent the moon and sun are also good luck so Dean isn’t a total jerk throwing yellow into a pile. It’s more just considered a general luck, depending on what color you give/what goddess you want to cater towards.
I also tried to show it around the eclipse festival that there’s a lot of good will to demon slayers and they’re usually always being handed flowers or decorated by the public. I’m sure piles of pink flowers outside their fortress is pretty usually. They fight demons for scraps, they deserve it.