Get free program called Paint.net, it's quite easy to use and figure out.
Magic tool is thing in the photo editing programs that let you pick area of similar color unlike paint that let's you "bucket bomb" only exact shade of color at once.
You can choose tolerance of the color choosing from only specific shade to to same color to similar color to all colors are the same (tolerance of 0-100%, the smaller the number the less it the color to vary).