[atareh]

yeah! we include the instructions on the website to go gray natively :). The app will have the features listed on the page.

[yumaikas]

For windows users, there's stuff like http://arcanesanctum.net/negativescreen/, that might be interesting to investigate and/or integrate into this.

[xaedes]

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.