The incomparable Lora Alexander posted an excellent video tutorial on how to virtually color-drape someone, and it’s one of the best videos we’ve seen talking about it. If you’ve never used Photoshop, it’s a quick 5-minute watch that shows you what to do. (Also, how to use it to color type people, it’s pretty cool!)
If you’re experienced in Photoshop, what she does is select a clear, high-resolution image with a single color top. Then she uses the Magic Wand tool to select her clothes and adjusts the color by changing the Hue, Saturation and Brightness control under Hue/Saturation.