I hope someone can help as this is driving me crazy...
At the weekend I got a new Macbook Pro Retina (Early 2015 model). Before connecting it to an external monitor, I upgraded to El Capitan and the device on its own is functioning perfectly.
When I connect it to my external monitor, a Dell U2412M, certain parts of the display are blurry. Some research indicated that it may be using the wrong colour space and using the well known script I generated and installed a new profile to force RGB. There may have been a slight improvement, hard to tell...
I then read about setting 'AppleFontSmoothing' and tried all available settings, again, there may have been a slight improvement. I'd pretty much decided to live with it. Until I previewed a jpg in finder and it looked like image (1) below.
I then dragged the finder window back to the built in retina display, and it looked like image (2) below.
Much smoother, but a higher resolution - no great surprise. Now, without changing anything at all, I dragged the finder window back to the Dell monitor and I was shocked to see that the smooth image was retained.
For information, the effect is not visibly different using either a mini-displayport to DVI-D or mdp to display-port cable.
Just to be clear, images 1 and 3 above are screenshot of the same window, on the same monitor connected the same way to the same machine. The only difference is that between these screenshots the window had a short visit to the built in screen.