Want to know the secret? It's 2 minutes work. Get into Photoshop, do a new project with your desired resolution, click on the gradient tool and and select spectrum. Drag your spectrum gradien from right to left within the background and then voila! You got a specrum for the RGB.
Inside the Wallpaper Engine Editor put this spectrum on top of your wallpaper of choice, select the Tint effect or Color and you get a wallpaper with the colors of your spectrum. After this, put your spectrum to move to right and stretch your spectrum image to your desired speed that you like the color to change.
Have fun!
PS: If you want the spectrum, don't google it... You won't find it... But I can send it to you if you want.
Just leave a PM.

Hey, your work is very good.
I wonder how you made rgb effect with both very low file size and clearly good quality.
I made rgb effects in gif format but it had large file size and terrible quality.