It is relatively simple, you should just use
First of all, hover over the label before the color you want to change in the inspector. It would show a tooltip with info about the property name. In this case, it was
To make it with
add_color_override, take the right-most part of the property name. E.g.
font_color. Then, use it like this:
Making it with
set is just a bit easier (and a character shorter):