If you connect an external monitor to your MacBook and the text looks very blurry (as in the image below), this link may help:
http://www.ireckon.net/2013/03/force-rgb-mode-in-mac-os-x-to-fix-the-picture-quality-of-an-external-monitor
It worked for me with my 2015 13-inch MacBook Pro and an HP LP2575w monitor connected via HDMI under El Capitan (10.11). The problem did not occur when the monitor was connected via DisplayPort, but I occasionally need that port for another PC connected in parallel. I had also successfully applied the fix before under OS X Mavericks (10.10). Unfortunately, after updating to El Capitan, it had to be applied again.
