A crow quill dipping pen is my first go to solution for these situations. The watercolors or diluted acrylics do the trick,the KEY is pointillism. Do not apply the color in a solid manner. A small amount of pigment in the quill is used to DOT the area. You 'sneak up' on being finished. Less is more,go in conservatively. More can be done in stages of alternating colors,just like the impressionist master painters. The eye will mix the colors and it will be a better cohesive match by letting some of the watercolor paper shine through.
Just for general knowledge this trick works super for posters with black areas that have been crescent damaged or some such trauma. The crow quill (a very tiny straight quill, comes with a cheesy plastic handle and everything for about $1.99) with Pelican Brand India ink ( it is blacker and doesn't smell bad) and the pointallism technique will work miracles making the white paper disappear ,as the ink seeps into the broken paper fibers only.
Get permission to touch stuff up and go for it. In 18 years I have never had a nasty outcome from a client I tried to help...although I do have heaps of art production under my belt...so I apologize if I make this seem simple.