As with car paint mixing systems where the process is similar, ultimately how good the match is is down to the eye of the person mixing the paint. Some are better than others.
My wife, or rather her workers, do not paint houses. Generally they are working on much larger buildings and structures. Right now they are working on an EGAT power station. Often large corporate customers have corporate colours and will provide the paint formula and specify the supplier it is to be sourced from.