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This year I've been able to provide my cacti with a cold winter's rest and a lot more sun in the growing season. As a result I've been getting more flowering plants, which makes me a very happy man. I've recently had a flowering Gymnocalycium bruchii and the flower had a beautiful pale pink colour. I compared the flower to other images of flowering G. bruchii and was wondering where the variation in intensity in flower colour comes from? Is there literature on this subject?

Mainly genetic variation.

When visiting a grower, someone from our local cactus society told me -regarding a flowering Stenocactus- the colour of the flower was pale because it was hybridized a lot. I'm not familiar with this, but is this what you are refering to? So soil, exposure and fertilization have little infuence on the shade of colour of a cactus flower?

I agree with Spence, although I do have to wonder if different fertilizers can have an influence on flower colors. Since I recently changed from the Dyna-Gro 7-7-7 I had been using for years to a more comprehensive fert regimen (Oscar, you know what it is), this will give me the opportunity to see if there are any appreciable differences in flower colors. I haven't seen signs of that yet, but I can tell you this -- compared to the relatively high P in the 7-7-7 (N = 1, P = 0.44), lower P in the new regimen (P = 0.35) has led to more prolific flowering on some of my cacti. Those are the spring bloomers, and I'll see if the summertime bloomers do it too.

Well, it's the first year I've actually had so 'many' cacti blooming, so I can't compare to previous years. I thought (in habitat) certain enviromental parameters could trigger genetic differences. Are there studies why flowers have the colour they have?