Using a high definition display on Mac OS X can be a pain to set up due to the fact that the system recognizes the monitor as a television and therefor forces the monitor in the YCbCr mode. There's a solution for the problem called EDID patching which forces the monitor into the RGB mode.
First of all, when are you using a version of Mac OS X which protects your system (>=El Captain), you have to disable the System Integry Protection before we can continue. To do so, you have to boot your Mac into the Recovery mode using the shortcut CMD
+ R
right after the startup chime. Select a language, go to Utilities in the menu bar and open up the terminal. Now insert the following command and reboot your Mac:
csrutil disable
reboot
Please keep in mind we're disabling a security feature - make sure you know what you're doing