Use the Magic Eraser Tool (click on the illustration below to enlarge it) shown on the Tool Palette, to erase large areas of the same color. For example, if you wanted to erase all the blue in the background of this illustration, you could use the Magic Eraser. Just pick it, go to the blue color and click.
Practice selecting and erasing.
Create a Photoshop document similar to the one below.
Select an area on a shape using the one of the selection tools_lasso, polygonal lasso, magnetic lasso.
Erase, recolor, etc. Select>Deselect.
Select an area using the Magic Wand Tool.
Erase, recolor, etc. Select>Deselect.
