Please refer to here for issue background details. Below is just steps for El Capitan running on Retina MBP with Dell U2410/U2414 monitors.
- Download the patch-edid.rb script from Andrew Daugherity’s improved patch-edid.rb script. Put the script on Desktop.
- Connect the external monitor.
- Type “cd Desktop; chmod +x ruby patch-edid.rb; ./ruby patch-edid.rb” in Terminal.
- A new folder will be created on desktop. Move it into the “/System/Library/Displays/Contents/Resources/Overrides/” folder. If prompted if you want to overwrite an existing folder, consider backing it up first.
- Restart Mac. The monitor should changed color mode to RGB.
- Optionally, Adjust the color profile inside Mac OS and/or monitor presets as needed.