The Magic Wand tool lets you select regions of similar color. This may be useful if you need to select an oddly-shaped region of one or more colors - impossible to do by the Select rectangular region tool and possible, but time-consuming with the Lasso tool.
To select a region of similar color, choose the Magic Wand on the toolbar window and click with your left mouse button on the desired color in the editing window.
To select multiple regions, hold the "Shift" key down as you select the second and all the next areas. You can then work with all selected areas simultaneously.