[musebox35]

That is likely. Another factor that came into my mind is the gpu using less power due to simpler computations. You can store less data for grayscale, so you need to go over less pixel data to do effects etc. Whether accessibility controls achieve this or not would be implementation dependent I guess.

[Dylan16807]

Even with the best GPU optimizations, most of the data will be processed in full color and then tossed through an extra pass at the end. More likely is that all the data does that.