First off, I wouldn't suggest using rainbow player mod as it is pretty hated, but whatever, your choice.
RGB values are color values. It's saying how much red, green and blue to put into the color you're making. The "1" at the end of the $Pref::Avatar statements is saying that it's not transparent.
Basically, if you know your basic color mixings, you'll be alright. If you don't, go into Blockland, press "Player" on the main menu and click "Colorset". Play around with the sliders awhile to familiarize yourself with them.