https://programming.dev/post/21199936
I've tried this quickly and you can use pango markup to change the text color in the label.
Here is a snippet with a label and a button that changes the text color from blue to red.
let label = Label::builder().label("<span foreground=\"blue\">Blue text</span>").build();
label.set_use_markup(true);
tool_box.append(&label.clone());
let button = Button::builder().label("Change").build();
button.connect_clicked(move |button| {
println!("Moved");
label.set_label("<span foreground=\"red\">Red text</span>");
});
tool_box.append(&button.clone());