Trying to use a fan deck or even the original paint you have leftover when you painted your walls say 2-3 years ago or more just will not cut it. The walls quickly oxidize and develop dirt on upon the wall surfaces.
I use a heavy duty utility knife and score the outer layer of sheet rock an area about 2 inches x 4 inches. Do this twice to make 2 samples (in case the paint store messes up or runs out of room comparing drydown samples). I try to get samples representative of how most of the walls look. It would do much good if selected an area that has more dirt, etc. on it, because the new paint would not blend in properly with the existing lighter wall areas.
Then I take the samples to the paint store and have them do a scan analysis of the paint chip.
Often times we discuss what needs to be tweaked and add either more of the regular colorant that comprises the new paint or sometime add things like raw umber or black to tone it down, mute it or darken it.
If done properly the match can be a pretty darned close match and certainly suffice, covering up unsightly marks or dirt that will not wash off or patching dents. And save some money.