Artists often check black and white greyscale for values. It is very critical for having an accurate rendering. The current option would be creating a
Color layer on top of everything but it would be very nice to have a greyscale toggle that can be assigned with a hotkey.
The Photoshop Proof setup also gives the ability to sample colors of the actual layer instead of Black and white values while viewing and painting at the greyscale view.
For more info about greyscale:
http://www.artofscholes.com/checkingvalues
Thank you.
