Our display code in 4.15 would mostly be older than the version of amd-staging-drm-next that you tested earlier. Would you be able to give amd-staging-drm-next a spin again and see if it's still an issue there? We've had some changes in our color programming recently. Note, though, that there's a different bug currently, described by https://bugs.freedesktop.org/show_bug.cgi?id=105083#c11.