Just a couple of notes:
1 - If you have a question about a specific plugin, it always makes sense to post in the thread for that plugin.
That's where you're going to possibly contact the author(s) and any users who are following the thread because they use the plugin.
If you open the plugin in a text editor, the colors used are defined in the drawStaticGauge and drawAnimatedGauge functions. If you search for it, you'll find for example line 487:
this.drawStaticGauge(x, y, width, rate, this.textColor(0), this.textColor(8), "exp");
You can see the 5th and 6th arguments are the start and end colors the gradient is made from. If you use the textColor function like that, it's referencing the colors in your Window.png file.
All of the different gauges are right around there, you can scroll up a screen and see drawing HP, MP, etc.