How do i do that?
You can use screen capture to get image. And ask a friend who familiar with edit image (Photoshop would be great) or an artist friend to recolor the part you wanted to. This applied if you want to use it as a ref sheet or some sort like that. It won't change ingame however. You can try fumble around with the program yourself too. Ibispaint for mobile and paintnet for PC. If you feel like it, you can use Photoshop, Paint Tool SAI or Clip Studio Paint.