You'll see why in a minute. Before you do that, you should save a backup copy of the file. Photoshop own file format is called .psd, and it's best to save Photoshop files in that format until you are ready to make the final save. At that point you would convert the file to a .gif or a .jpg, and save it again. Saving periodically will give you a backup copy of the file at various stages.
Nothing is worse than having to recreate an entire image file from the beginning.
If you save a file halfway through the process, you only have to recreate half of the file. Coloring and highlighting the button is very easy. Click on the Select menu, and choose Inverse. When you first selected the button, the area inside the marching ants was the selected portion. By selecting the Inverse of that, you selected the outside.