Okay so I think I can help you, I upvoted and have been checking this post for the past hour and no help has come, so I went with the approach of "If there's no help, then help yourself"

I wanted the button black but, it should be easy to have any color provided you know the mix which can be found by any standard color picker;

Here's what you do :

1.) In rainmeter, find NXT-OS, Open it, Then Open GameDrawer, Then select GameDrawer.ini, then press edit in the top right hand corner of rainmeter.

2.) Scroll down till you see a section labelled "Meters" the first two paragraphs are called [LoadingColor] and [LoadingColor2]

3.) In each paragraph there is a single line called "SolidColor" this is the actual color of the button, by default it was red, so for me it looked like this SolidColor=255,0,0,225 all this is is the color based on a RGB color picker and its brightness I assume, using a color value off of any color picker, change this line in each paragraph and your button should be the color you want.

I'm a total noob at rainmeter still, but hope this helps.