Luc and Antony, what you said is correct.
There are clones which have dark or light flowers and they will always flower darker or lighter than other plants. If the weather is cold during the development of the flower Cypripedium calceolus will be flower a little darker generelly. If the plants have stress they will flower lighter (even the dark flowering Cyps). I have never seen an influence of the winter temperatures on the colour of the flowers.
After the very long winter 2005 with a lot of snow (flowering time was in a cold June) I have found many very dark flowering calceolus and a lot of very light (yellow, green,...) plants.
All this is valid for the nature!
