https://bugs.freedesktop.org/show_bug.cgi?id=70934
--- Comment #11 from Alex Deucher agd5f@yahoo.com --- (In reply to comment #10)
To clarify: I did "git checkout eccea7920cfb009c2fa40e9ecdce8c36f61cab66", tested the kernel and got that colors problems. On top of that commit I simply did "git revert eccea7920cfb009c2fa40e9ecdce8c36f61cab66", re-tested, and colors were OK again.
So it's definitely matter of eccea7920cfb009c2fa40e9ecdce8c36f61cab66.
Sounds like your EDID may have a buggy bpc setting then. If dithering doesn't help, we could add an EDID quirk to fix the bpc.