There are two tools available for filling an area within your image:
You may use this to fill an area of one or similar colors with another color. For greater discussion of how to use this tool, please visit the Paint Bucket Tool section.
The Gradient Tool allows you to draw gradients (a gradual blend of one color to another) in several formats. With its transparency mode, you can also "fade" or "blend" two images together. For greater discussion of how to use this tool, please visit the Gradient Tool section.