If you want to paint your walls a different color to match a specific room in your house, you'll need to get an exact match from the paint store. Unless you have a photographic memory, you likely won't be able to find an exact match without using a sample. Therefore, you need to remove a paint sample from the wall with the color you want to match.
Locate an obscure section of the wall, such as behind a piece of furniture, and hold the heat gun two inches away from the wall. Apply the heat -- heat loosens the bond paint has to wood and drywall. Rotate the heat gun in a small circle; do not hold it stationary for more than ten seconds, or you risk scorching the wall.
Watch the paint. When it begins to bubble, turn off the heat and get your scraper.
Hold the scraper at a 30-degree angle from the surface of the wall and push downward. Hold the heavy-duty plastic bag below it to catch the falling paint. Use the largest resulting paint chip as your sample.