The process for colour matching is actually not that difficult. In order to get an accurate or similar shade of colour, it will require a paint sample. The best place to get the sample from is the wall behind a door. Cut a small square piece about the size of a 10 cent coin. Bring this sample to the paint counter and they will do a colour paint match. At this time colour photos on your phone can't be used as a reference when mixing colour tints.
Once you have your paint matched, remember to purchase a Poly Wall Repair Kit to repair the little square you cut in the wall. Please remember to follow the instructions printed on the kit. Once the repair is dry you can then brush some paint over it.
The paint specialist in the store will do the paint base picking for you. All you need to do is to bring the sample and ask for a colour match. After the paint match, the team member will do the rest for you. They will pick the base, add the tint and place the can in the paint shaker. They will then do an actual physical match against the paint sample you've brought in.
Please remember that some paint shades behave in a certain manner. When fresh paint is placed on existing wall paint it sometimes looks like a different shade. You must give time for the paint to dry to see its real shade. If the paint match is very close you will not even see where you painted. - EricL