I have this same problem. I took my MSI 5770 out of another working hackintosh and it gave me this issue.
This is the issue described in a youtube video. Shows windows dual boot working fine then Sierra. SO weird!
This did not switch my issue, in fact. I downloaded the Display Maestro and the resolution actually shows mine with Millions of colors already.
I also have a LG Ultrawide 21:9 monitor, could this be it?