Apple - External Display and Blurry Fonts on El Capitan

The location of the Overrides directory changed to /System/Library/Displays/Contents/Resources/Overrides/. Note that you will need to disable System Integrity Protection to add files there.

It is a good idea to check whether the monitor in question is already using RGB before the above is attempted as it may not achieve any benefit. To check: Utilities/System Information->Displays and look to see if it is using RGB (You can also check in System Preferences->Displays-> Color->Open Profile->Header->Space) if so then your problem lies elsewhere.

Another possible solution is to try altering Font smoothing options in System Preferences->General'. Or if you're seeing problems in a particular app - e.g. In Preview go to Preferences->PDF and untick 'Smooth text and line art'.