In general, purple inks (and red inks) tend to stain pen parts, although Skrips purple may a better option from the staining standpoint.
https://www.fountainpennetwork.com/forum/topic/305572-purple-sheaffer-skrip/
I think it will be challenging to find a water resistant purple ink from the manufacturers generally considered safe for vintage pens. JHerbin Amethyste de L’Oural has been reviewed as having some water resistance, and is made by a manufacturer often considered “safe” for vintage pens. It is, however, a “glitter” ink, which many avoid using in any pen. A potential way around this is to allow the glitter to settle to the bottom, then either decant to another bottle, or draw ink only from the upper portion of the bottle.