I used the Show FPS effect to troubleshoot a KDE problem, however there is no documentation explaining the color and what it means when it peaks!

Right, even I have to look into the source code to understand what it does. In general: don't use it. It's a tool to debug problems in KWin and nowhere else. In fact it destroys the benchmark just by running. So if you want to troubleshoot a problem in KWin I am willing to help, if somewhere else, it is useless.