I've been using this '1/8" method' to clean the brushes I use for oil based
primer. The primer is Zinsser Cover Stain, just white, no tint.

https://www.youtube.com/watch?v=DDvDbKBTbic

I collect the used mineral spirits in a jar and let the primer settle out
so I can recycle the mineral spirits and use it to clean the brushes again.

I've found that instead of the mineral spirits being clear after the primer
settles to the bottom of the jar, it is sort of an amber colored clear. I
don't think it's a matter of not waiting long enough because I've gone weeks
without touching the jar.

The recycled spirits seem to work just as well as new spirits for cleaning
brushes, but I'm curious as to why it is no longer clear. What is in the
primer that would cause the spirits to turn amber?