Via ThoughtCo, I happened upon this article on How to Fix Mistakes & Make Changes in Watercolor Paintings written by Lisa Marder. Lots of helpful ideas here! I was especially interested in the following tip from Marder:
"Mr. Clean Magic Eraser is an amazing cleaning product that looks like a white sponge and that when moistened is a stable polymer abrasive that acts like an ultra fine sandpaper to remove stains, dirt, grime, and even paint pigment from in between the fibers of paper!"
"Be sure to use the Original brand, because later versions have additional chemical cleaners in them that aren't good for your paper or painting. The original sponge, though works purely physically. When damp it easily lifts the watercolor paint from the surface enabling you to go back in and repaint the area you've erased."
Read the rest of the post for more great ideas on correcting watercolor painting mistakes.