It's actually a mix of two brands. Here's what I posted on TFW:
"It's a mixture of Citadel Genestealer Purple, several drops of Createx fluorescent violet, and if I recall correctly just a very, very small drop of createx blue. The two brands aren't 100% compatible but it works fine if it's mostly Citadel in composition.
I'm sure you can use purely Createx paints to achieve the color, but it's an airbrush paint with some amount of transparency that became really dark when I handbrushed on a few coats. I didn't have any createx white on-hand to compensate, so I started over with the Genestealer Purple to brighten it up a bit and add a more opaque base for better coverage."
I actually hadn't thought to try a paint pen.