When deciding which colour difference calculation to use, consider the following rules:

1. Select a single method of calculation and use it consistently.

2. Always specify exactly the equation used, illuminant and observer. Model of instrument used should also be specified.

3. Never attempt to convert between colour differences calculated by different equations through the use of average factors.

4. Use calculated colour differences only as a first approximation in setting tolerances, until they can be confirmed by visual judgments.