Change RGB color to a button permanently

Hi. I’ve got a small but probably very simple problem.

I’ve got 5 images that appear in order when I click
for example image nr 2:s button “b2”

The thing is. The buttons are just simple grey shapes with no names on them, so I want the button clicked to change it’s RGB color to a darker grey and to stay that way until I click another button.

How can I do this?

Yours sincerely Marcus Brunström