Invert colours on Dual-Screen / Dual-Monitor setup?

Here is a small utility program that does what you want:

https://github.com/zoltanp/xrandr-invert-colors

One could try fiddling with negative gamma-values and brightness-values < 1 to generate a negative gamma-ramp on a specific output.

xrandr --output [output] --gamma -0.5:-0.5:-0.5 --brightness 0.1 

this suffices at least to me and works out of the box, but does not take into account any existing gamma-curves for an output. might be worth a shot. Greetings konrad

Try xcalib -d :0 -invert -alter

The -d option refers to the device, and it worked for me when -s didn't