A professional painter should keep in mind a number of factors when selecting paint for a job, the most important rule is to never choose paint for the client. Let the client do it, and carefully explain how different forces can affect the final color of paint.
This rule should be ironclad. Many a frustration has resulted from the painter's picking the color of the paint. Once the paint is swatched, it will hardly look the same when its is on the house. The normal procedure when a paint is mixed, is to dab a little of the mixed paint on the color chip and than wait for it to dry. The chips used to match are paint, so they will likely be very close.
If a chip has a flat or eggshell sheen, the dried dab will appear different, and sometimes more dramatic because of the way it reflects on the light. Paint will change its color depending on the amount of the light in the room. Those factors that affect paint color are surface porosity, the light reflectance of colors from adjacent objects.