>>7627381
They are much cheaper per ml and not designed to be lightfast.

Nicker and Shinhan use Gum Arabic as a binder. Cheaper varieties may use dextrin.

The best poster color tutorial is in fucking THAI but the translated captions somewhat work:
https://www.youtube.com/watch?v=bnfPtEWWxks
It's 1/6, full playlist here: https://www.youtube.com/playlist?list=PLQ1tctKNWdwo3aoUwJPQe36ZQlMsbgX1Y

He has 2 other playlists of it but I haven't watched them. There's some additional info on using them, derived from those videos and experimentation, on this dude's blog:
https://monisawa.blogspot.com/2014/02/shinhan-poster-paint-part-1-of-3-journey.html
https://monisawa.blogspot.com/2014/02/shinhan-poster-paint-part-2-of-3-my.html
https://monisawa.blogspot.com/2014/02/shinhan-poster-paint-part-3-of-3-using.html

>>7599968
Cel era anime backgrounds were mostly painted with poster colour.
>>7600005
>>7600012
Besides the fact that most peoples' homes in the late 20th century had a lot of earth tones...

Cel paints are already desaturated. Making the background more desaturated helps the characters read better. Browns are desaturated oranges, and blues contrast with the warm skin colors.
But mostly it's a "paint what's there, nothing fancy needed" - these people were on deadlines and you're showing houses, not space ships. Take screenshots from Macross, Harlock or Gundam and you have less browns.

>>7599999
>>7600000
Jesus christ.