If you’re using Photoshop’s Healing Brush, and the results are too dark for your taste, you don’t have to undo and start over, simply choose Edit>Fade Healing Brush and then lower the Opacity setting in the dialog box. The Fade menu item works on many tools and filters, so I encourage you to take a trip to the Fade menu item once in a while to see what you can do. Keep in mind that it only works on the last operation you made.