I like to use CSS plugin (you can find it here on forums). Create css file and import it to Files folder in Construct project. Then in CSS plugin write file name of your css file (stylesheet.css for example).
Now if object supports CSS styling it will have option to set it's refferencing ID (buttons and text inputs have, probably progress bar have it too).
Let's say that you entered progressbar as ID, in CSS file you must enter
Try searching google for all CSS parameters and how to use them. Here, i think, that you need color, or background-color parameter to change.