Superimposing layers of transparent color images over a grayscale image can be quite effective and easy to boot. Open the grayscale image you want to use and be sure to convert it to RGB color mode by choosing Image>Mode>RGB. That way, the image is ready to accept color. Open other color images containing the colored elements that you want to layer over the grayscale image, and copy and paste them onto new layers above the grayscale image. Lower the Opacity setting of the colored layers and voila! The grayscale image now contains interesting colorful elements.