https://homeairquality.org/how-to-prevent-pink-mold-in-humidifier/
"like any mold species, pink mold thrives in moisture and darkness."
So it doesnt matter if your stuff is "black". Its growing because unlike water lines, you are "filtering" a crap ton of unfiltered air through your tubes.
This was my hypothesis too. "It doesnt matter if you use distilled water and black tubing". If you dont clean and have anti bacteria, you are going to get the pink stuff no matter how pure your water is. Your air is full of lots of yummy stuff for the bacteria to eat
I tried using solid hydrogen peroxide (the main ingredient in oxiclean) that is used to remove algae from fish ponds. It didnt work...