You can add spot color to selected areas of an image in different ways with varying effects. For instance, you can apply spot color to part of a grayscale image so that the selection prints in the spot color rather than in the base ink. Because spot colors in Photoshop print over the top of a fully composited image, you may also need to remove the base color in an image when adding spot color to it. If you do not remove the base color, it may show through the semitransparent spot-color ink used in the printing process.
You can also use spot color to add solid and screened blocks of color to an image. By screening the spot color, you can create the illusion of adding an extra, lighter color to the printed piece.