Nicked window trim, or trim that is scratched or otherwise compromised, needs just a dab or two of paint to remedy the marred surface. Unfortunately, if you don't have the original paint, you are up against a problem. Even if you do have the original can, it possibly is dried up or too old to use. If you have a very good eye for color, and experience color mixing, you can purchase tints and try to match the trim color. The easier way is to work from a swatch book. Most swatch books have very similar colors. Even if they are not precision matches, for a small speck of trim it should be close enough.
Use the swatch book to determine the hue of the window trim paint. Do this in good light. Daylight is best.
Purchase a small sample jar of the hue at the paint store. This avoids a bigger expense, and you will still have plenty of paint for small repairs. Get a semigloss finish, which is standard for trim. If no sample is available, purchase the smallest quantity possible.
Dip the brush lightly in the paint. Dab it onto the touch-up spot.
Rub the paint in with the rag. This distributes the paint over the area around the touch-up spot, making the repair harder to see.
Repeat the application and rubbing process several times. Finish with a very light coat on the touch-up area for complete coverage.