Why does thunderbolt to displayport cable result in video with color bleeding?
Wow. So. Apparently OSX is treating my display as if it were a TV, and is only sending YCbCr color space instead of full RGB color space.

Found this article by Robert Pill that indicates the problem, and how to fix it:

How to force RGB in Mac OS X


Download the patch-edid.rb script from the forums thread above and
put it in your home directory.
Connect only the external monitor(s) in question (I closed my MacBook
lid, for example). The script will make override files for any
connected monitor. Type “ruby patch-edid.rb” in Terminal.
A new folder will be created in your home directory. Move it into the
“/System/Library/Displays/Overrides” folder. If Finder tells you that
you are overwriting an existing folder, consider backing it up first.
Restart your computer, enjoy your monitor.

To undo the changes, either delete the folder you had copied to the
Overrides folder (if it didn’t already exist) or replace it with the
folder you had backed up.


Also, one of his commenters made an improved script, available here.