El Capitan system font blotchy on non-white background [Non-retina monitor]
I've found a solution for anyone who has this problem in future. Turns out this problem was hard to communicate as even the screenshots I posted appear fine for most people.

OSX machines were detecting this Samsung LCD monitor as a TV, therefore setting the color profile to YCbCr colour space instead of RGB. It turns out that this problem reared it's head a lot with Dell U2410 monitors in the form of blurry fonts.

The solution is to override the EDID (Extended Display Identification Data) for your monitor so that OSX outputs in RGB mode.

The solution for doing this is detailed at, but I will repeat here.
http://www.ireckon.net/2013/03/force-rgb-mode-in-mac-os-x-to-fix-the-picture-quality-of-an-external-monitor/

Disable System Integrity Protection


Restart Mac, as booting, trigger recovery mode with Cmd+R, then launch Terminal.
Enter csrutil disable and press return.
Restart Mac and login.


Generate Monitor Color profile override.


Download and save this ruby script https://gist.github.com/adaugherity/7435890
execute in terminal ruby saved_edid_script.rb
It will output a folder in your current directory.
Take this folder and place it in /System/Library/Displays/Contents/Resources/Overrides (El Capitan location, folder is a couple of levels up in older OSes, backup any folder with existing name)
Restart Mac


Re-enable SIP.

Same steps as "Disable SIP" but with csrutil enable