First you're going to start off with a new image.
As always you can follow my specifications on size or do one of your own sizes.
Next you will need to flood fill with a gradient, I used the "Autumn" gradient.
After you have flood filled the image this is what it should look like.
Next you are going to use the colorize function.
When you have the colorize dialog box up you will want to enter in the specifications that I have marked.
After you click ok your image should look like mine!
I hope you enjoyed this little tutorial. If you play around with the specifications in the colorize feature you get some neat results. Try doing this with different gradients and lets see what you made!
Like I said, as I play around I will make more tutorials.