One of the real benefits of working digitally is the ability to be flexible. Want to change the color? No problem. Want to move something around? No problem. It takes a different mindset than traditional drawing, but if you can wrap your head around it your work will improve. This video explores the use of "copy merged" to modify a piece of nearly-finished concept art. Traditional wisdom says that once you've made it this far into a painting it's too late to move things around, but Photoshop makes it much more possible! Though it's always better to work out the problems in your illustration earlier in the process (ideally in the thumbnail sketch), you will occasionally need to make last-second course corrections.