System Information
Operating system: Mac OS X Catalina 10.15.1 (19B2106)
Graphics card: Radeon Pro 5500M

Blender Version
Broken: 2.81a
Worked:

Short description of error
No matter which color is selected for line, it appears some shade of green with some kind of dithering/noise artefacts. The shade of green changes slightly depending on color and viewport shading, but not much.

Exact steps for others to reproduce the error
Create new 2D Animation file with 2.81a on Macbook Pro 16" and draw a line.