On a sheet of freezer paper, create all kinds of different sized raindrops, then cut out with scissors. Place raindrops waxy side down on the front of your shirt and iron in place to create a temporary bond to the fabric. Make sure your work surface is covered with a plastic tablecloth, then begin spraying a light coat of gray across the top of the shirt, working your way two-thirds down the shirt and applying a lighter mist of color as you get towards the bottom portion. Remember to spray the sleeves too!
Keep applying several light coats of color versus one heavy coat to help prevent color bleed, and let each coat dry before applying the next. When you are happy with the color of your shirt, let it dry then remove the raindrop cutouts.