paint in
To color or fill in some blank space with paint. A noun or pronoun can be used between "paint" and "in." I brought in a bunch of black-and-white pictures of cartoon characters for the kids to paint in. There's a big streak where the table scraped against the wall. I'll need to paint it in this weekend.
paint something in
to paint something extra onto a painted area. I know that there is supposed to be a big white spot here. We will have to paint it in. We will have to paint in the spot.