Mathematical Morphology relies on the notion of ordering. For colour image processing, this question is relative to the colour space choice, to the colour distance defined in the colour space, or to the importance of each colour axes in the colour representation or statistical colour organization. For twenty years, more than 60 different proposals have been developed to express such orders, but how to choose the right ordering form, on which criterion to ensure the stability of the morphological behavior? In this paper, we investigate one criterion and we apply it on some approaches in colour Mathematical Morphology.