Hi Wiebe,
The variation in spine colour is interesting, isn't it. I have noticed this in many species, and also have seen many plants offered for sale which have (for example) the species name and var. bruneispina, or var. flavispina. As it is a natural variation, such named varieties are to be ignored.
M. guelzowiana has such great flowers, thanks for posting lovely pictures.
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Chris43, moderator