I've been growing 'Amethyst' for a couple of years. It does have a nice 'blue' mauve shade, but the colour morphs according to the time of day! It has a blue tinge in the morning, turns magenta in bright mid-day sun, and turns back to its blueish form in the evening! I've never seen this colour changing habit reported anywhere, but have just seen an American website where the same thing is reported of the purple variety 'Nicky'- with a sequence of photos to prove it! It is normal for flower colours to change as the flower ages, in a one-way process. But it appears that with phlox that the flowers can shift from a blueish tinge, to magenta and back again. It appears to be the strength of sunlight which triggers it, so perhaps planting them in a shady area might help. But it does make it very difficult to know what colour you are buying, as it is not simply the quality of the illustrating photo that has to be considered, but when in the day the photo was taken!
I too am looking for a really deep purple phlox. My family used to have one years ago, and the colour didn't seem to change. I'd love to get another one. but the deep purples seem hard to find these days. It must have been one of the older varieties- any suggestions please?
Regards,
Andre