How do you switch between normal colour and grayscale in Photoshop?

I tried Stephen’s suggestion and it seems to work fine.

It is important for you to set up Proof Setup using the correct grayscale profile. Photoshop comes with several, and the OS might have a few more, and they represent different output conditions. For example, if your work is for the screen and never for print, do not select any of the Dot Gain grayscale profiles, instead choose something like one of the Gray Gamma profiles, probably Gray Gamma 2.2 unless you are targeting specific output such as broadcast video (2.4).

Also, since you mentioned checking values, if you want to do that precisely in addition to visually, use the Info panel. Set one of the sampler readouts to Proof Color to see the value of the pixel under the pointer. It reports the value after converting the color values to the current profile selected in Proof Setup. (In my example I chose Grayscale, but later realized that probably displays values in the currently active grayscale working space set in Edit > Color Settings, not the values for the currently active Proof Setup. So choosing Proof Setup instead of Grayscale is probably better for the purpose in this thread.)