Here is a nice portable tool to generate random wallpaper with color gradients and display it on your Windows desktop with just one click! It is called Random gradient wallpaper generator.
Create random gradient Wallpapers with Randomize
It is a simple to use wallpaper generator to create desktop backgrounds.
Randomize assigns random colors to all four corners.
Set as Wallpaper – saves the picture to My Pictures\GradientWallpaper.bmp and sets the Windows Desktop wallpaper to this file. It will overwrite the file if it exists.
Save colors to text file – appends the RGB values of the current colors to the “colors.txt” log file in the program’s directory, in case you liked the colors and want to save the colors, not the image.
You can also click on each of the four corners and pick any color yourself.
There are a total of 79228162514264337593543950336 possible wallpapers that can be generated this way.
You can download it from Codeplex.