In the image editor add a new image and with default settings just do a red line paint over a green line paint.
Dark zone appear in the falloff between red and green. it should not be there and should stay clear, not dark.
I don't know why, but it looks like a color space issue.
dark falloff not ok in blender.
{F7649279}
clear falloff ok in gimp.
{F7649281}
blender 2.8
Windows 7