sudo apt install screenruler

Helps to measure pixels on a display. If you set your physical resolution correctly, you can also measure in millimeters.

Get colours of your screen.

sudo apt install gpick

Click on one of the 6 hexagons from the top left, and then move the cursor to a pixel, you want to get the colour from, and then hit the space bar. If the source colour is noisy, you can add averaging filters with different weight curves. This is called „oversample“ and „falloff“. On the right side in the list you can rename your colours