My external monitor goes grayscale on the non moving parts of the screen (mid-2014 Macbook Pro Retina) I have a mid-2014 Macbook Pro Retina 15". I also bought a 27" Acer K272HUL and I've connected the 2nd monitor to my Macbook via HDMI. 

So now I have 2 screens, my 15" and my "27". I usually play videos and streams on my larger 27" and periodically the stream is in color and the rest of the screen is in shades of gray. More specifically my external monitor goes grayscale on the non moving parts of the screen. 

Is that normal?
Turns out it's not a Mac thing -- it's a "feature" of the Acer monitor. In the monitor's menu, disable "Detect Video." Confused me for a bit, was glad to discover I'm not going crazy.