Forgot to mention that the old laptop was attached to these same two monitors through the Dell docking station, and both monitors dimmed as expected with f.lux. Also, when connected to the dock, the laptop is generally closed, so the laptop monitor is turned off, and the Dell monitor (Monitor #1) is the main screen. Uninstalled and reinstalled f.lux, and expanded the color range again. Still doesn't work.

Huh! Good to know - I guess that's a tick in the "pro" column for upgrading to 10.11, which I'll do once I can make sure it won't cause other problems elsewhere... cheers, j

We have noticed that the NVIDIA card handles color profiles wrong on 10.10.5. This appears to be fixed on 10.11.