The primary measure of visual colour difference is “the delta”, which is a measure of the difference of colour and is set at a tolerance between a master colour swatch and a production batch. The delta is also a standard measure for colour computers and are so well defined that computers should be recalibrated every three months.
A client may sometimes require a tolerance for colour variation because even small differences can result in quite apparent visual colour differences, as some colours are more exacting than others, like white, for example. Another cause of visual colour difference is the light source under which the leather is viewed. Incandescent, fluorescent and LED lighting will each show colour differently; the wattage of the bulb and even the bulb brands themselves will have an effect on how colour will show. This is because each type of lighting produces a different tone of light.
Personal perceptions of colour may also differ depending on what amount of colour sensitivity the individual has. Being colourblind is an example of this.. The only way to see “true” colour is to use a Macbeth Lighting Box which uses a fine tuned, balanced system to detect discrepancies between colours.
Ultimately, if a precise match is essential, CFAs (or “cuttings for approval”), are the surest way for getting the closest match.