Apply strips of painter's tape to the edges of the painted sides so you can paint the space in between the two colors.
Repaint the area using a roller and the paint color you want for the space.
Use a tiny brush to paint over small specks or spots for mostly even paint jobs.
Apply strips of painter's tape to the edge of just one color if you don't want a space in between the colors. Paint the edges of the other color, let it dry, and then repeat for the first color.