yeah! we include the instructions on the website to go gray natively :). The app will have the features listed on the page.
For windows users, there's stuff like http://arcanesanctum.net/negativescreen/, that might be interesting to investigate and/or integrate into this.
I missed the grayscale option at first sight and discarded that solution. Thought it could only invert colors.
To make it clear: NegativeScreen has a grayscale option with default hotkey F11.
You don't need an app for android to do this system-wide (tho I could see why whitelisting would be nice). Just go to developer options-> Simulate color space-> Monochromacy