System Information
Operating system: Darwin-19.0.0-x86_64-i386-64bit 64 Bits
Graphics card: AMD Radeon Pro 580 OpenGL Engine ATI Technologies Inc. 4.1 ATI-3.2.24

Blender Version
Broken: version: 2.81 (sub 16), branch: master, commit date: 2019-11-20 14:27, hash: rB26bd5ebd42e3
Worked: (optional)

Short description of error
When working in Grease pencil, with Preferences>Viewport>Quality>Grease Pencil Multisampling set to anything other than "No MultiSample", strokes and fills are green, both in the viewport and in render. Sometimes the background flashes green while drawing a stroke. Turning the preference to "No MultiSample" restores the proper materials.

Exact steps for others to reproduce the error
Start a new file with the 2D Animation template
Check that Grease Pencil Multisampling is on any value other than "No Multisample"
In Draw mode, draw a line in the viewport