3D printed item magically changes color

Previous blog on 3D printed coil form:

original color

https://wb9kzy.blogspot.com/2023/03/elcie-rides-to-rescue.html

I was cleaning off the workbench top and came across the 3D coil forms - no I haven't done anything further except saw off the brass bolt head.  But the weird thing is the color:

the side is also more original

What happened there ?  They were sitting on the workbench under some paper and other junk.  Judging by where the color kinda went rusty it must be light related.  The workbench is next to a north facing window so there is only rare sunlight.

3D printing is now weird for me.

Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm