I’m not sure what you’re referring to but the easiest way to identity an element is to right-click it and “inspect” it in the browser (menu options may be called something slightly different depending on which browser you’re using). This will open the developer console. Use the style pane to see what CSS is applied, modify it live in the browser, then transfer your changes to the stylesheet indicated in the same pane. The appropriate CSS selectors will be given too.
Another less elegant way would be to screen cap the area of colour you’re looking for, and sample it in something like Photoshop. Then search for the hexadecimal colour value in your stylesheet(s). This doesn’t always work (depends on whether named, rgb or hex colours have been used) but often it does.
The main tool you should learn how to use is the developer console – that’s F12 in most browsers (if not right-clicking to invoke).
