Note: instruction below installs system-wide. better to use venv or pipx

Installation:

git clone https://github.com/madsl/Pro-IntelliMouse-Control-Panel
cd Pro-IntelliMouse-Control-Panel/
sudo pip install . --break-system-packages

Identify the connected IntelliMouse devices and note the index (0 in this case):

sudo intellimouse-ctl list
0. ProIntelliMouse @ "1-4:1.1"

Get the current colour for device index 0 and set some other colours. Note this requires int values of the colours rather than hex values.

sudo intellimouse-ctl get --color 0
color: 16777215

sudo intellimouse-ctl set --color 20000 0
sudo intellimouse-ctl set --color 30000 0
sudo intellimouse-ctl set --color 33000 0