While doing research to find the RGB equivalent of a color I saw on the screen, I came across a terminal application called grabc.
I can say that it will be very convenient, especially for front-end developers.
Setup
sudo apt-get install grabc
Usage
When you type grabc into the terminal, the cursor will look like a ‘+’ shape and the hex and RGB code of the color of the place that you clicked will be printed on the screen.
$ grabc#f55567245,85,103