invert colors on X (or xfce4)

You need the xcalib package:

xcalib -i -a

If you are like me and you have tried xcalib but you have encountered an error like this on debian:

xcalib: unsupported ramp size

Then you could use an alternative like: xrandr-invert-colors.

First clone the repository

git clone "https://github.com/zoltanp/xrandr-invert-colors.git"

After that you can install the build-essential package and you can compile the code with that commands:

sudo make deps-apt
make
sudo make install

After that inverting colors on the screen is simple as:

xrandr-invert-colors