"A gray soap"
We usually use "siwy" to describe hair color (especially of elderly people). Besides, you used wrong form of the adjective. "siwy" is a masculine adjective, whereas "mydło" is a neuter noun and should be described by neuter adjectives - so "siwe"
BTW, Geralt was called "Biały Wilk" (White Wolf): https://www.youtube.com/watch?v=YxQa9ozzq3c
well according to dictionary siwy is
- a very light grey color,
- a colour of the hair of the person, that has lost it's original colour and is now light gray almost white, with silver reflection. ( also a man with that hair colour)
According to my experience - every person whose hair color has changed and is now between dark gray and pure white can be describes as "siwy". also people with very light blonde hair or with albinism can be nicknamed Siwy/Siwa,
It totally depends; in the UK and the American South they use more terms, whereas the parts of the US I'm familiar with, I sometimes get called "redhead" just for having a red beard. Things like "auburn", "ash blonde", "platinum blonde", and "ginger" are starting to become a more common terms here, but I don't know if it's from UK English or from the labels on Loreal boxes.
Another unrelated anectdote - my babcia has a HUGE thing about making super mean jokes about "gingers", and I don't know if it's her sense of humor or genuine discrimination. Was this ever a thing in Poland, like it was in the UK, or is she just a special snowflake? Her family all has jet-black hair.
I have never heard one? Are they about red is not trustworthy, false, dyed fox, red witch? - those are the mean stereotypes here.
Blonde jokes are popular, about silly stupid blond girls.
I checked some joke sites. Most "red" jokes are brunette, redhead and blonde do something, brunette is a smart one, redhead is a bit silly, and blonde is very stupid. But the site has like 1000 jokes about blondes and 25 about redheads.