Hi Swim.
You will have a problem trying to match the colour exactly, due to various things.
On what machine the original colour was mixed (Mixing machines can vary slightly even using same colour codes)
Age of paint (Natural discolouration)
The sun as a huge impact on changing paint colours.
Dirt/Dust/Algae sticking to walls over time etc.
You will probably find in general that the new paint mix is usaully slightly lighter even if mixed exact, depending on age of original.
The best way would be to paint a full section, but that depends on the size of the section to be painted, and where you are painting.
Some companies will offer a scanning service where you take a flake of the paint into the shop and they will scan it, this can be hit and miss too.(Dont know if this is available in the area)
Other option is the human eye!!
Ive copied colours in the past using colour charts on swabs?? and matching up.
Dont even try adding bits of colours to lighten or darken, unless you are extremly lucky you will have little or no chance.
Im in the property maintenance business, from painting a small bedroom, to airless spraying a factory, so have tried many times over the years.
Hope that helps
Mr subby