I work in ProPhoto RGB. I converted the attached image to sRGB to post on this forum but the purple effect is present in the ProPhoto files
I often see this purple / blue thing in images containing very saturated blues. ( Stained glass - R72 G7 B230 ). I also often notice a loss of detail in these blue areas.
The ProPhotoRGB 72/7/230 is displayed as Lab 20⎢84⎢-125 in the color tool of Photoshop. Nice so far. Now convert your ProPhotoRGB 72/7/230 from ProPhotoRGB to Lab. One would expect that the Lab values remain the same. But they change to 38/55/-90... which is a violett.
ProPhoto converted to Lab: [attachment=15384:lab.jpg]
Exactly the same happens if you just desaturate your ProPhotoRGB blue... it becomes violett.
High saturated blues and ProPhoto are difficult to handle as ProPhoto contains colour values that can be set mathematically but that are outside of real colours. The highest saturated blue in ProPhotoRGB (i.e. RGB 0/0/255) is Lab 0⎢90⎢-128. Apart from that this is a colour far beyond any colour a monitor or any other real world device is able to produce it's moreover absurdity because a Lab value without luminance and saturation is by definiton black.
Your ProPhotoRGB 72/7/230 is even outside of J. Holmes' Ekta space...
But: didn't you see any clipping in the histogram?
edit... forgot about the "loss of details" in blues...
Depends: maybe (in this case I'm almost certain) it's just that you monitor clips the colours. If you move with mouse over the respective areas and look at the colour values you will see if there is any modulation or not.
But at these values in any case you will lose all modulation at the latest when it comes to printing.
Obviously C1 and RD take it into account and others don't (yet?).
C1 doesn't work with colour spaces from outer space. C1 is totally based on real world colour management - either way if there is a "bug" in Lab or not. We work with Lab (as PCS) ... so what!
If you like ProPhotoRGB for whatever reason you can process your RAW files to ProPhotoRGB TIFs from C1 - the camera profiles will provide the right input so that there is no risk that the imaginary colours of ProPhoto will be allocated.
It's always nice to learn about the "well behaviour" of certain colour spaces ;-)