It's just razzle-dazzle camo in action. Reminds me of a bit of fluff from 40K surrounding the Blood Axe orks. They saw imperial armies using camouflage and thought it was a great idea to copy, but missed the point somewhat and made their camo patterns in garish colours like purple, justified because, 'well, youz ain't eva seen a purple ork 'avs ya'. However, much like the collective belief of the orks that red vehicles actually go faster, and the way that the collective waaagh! energy works in-verse, it ends up being really effective camoflage
Purple works as one of the colours in a camo scheme (it works well with a darker green). Your brain will just not "see" stuff if it can't make out a decent outline of it. Just disrupting that outline or shape *enough* will do it. This is why yowie suits work, too. I had a unit of scouts (40k miniatures) done up in purple/green camo and I lost them in a piece of jungle terrain ( lots of shrubberies, no trees). Predominant colour was a bright fluoro green in the forest.
I swapped a mid-violet color for the red-brown in German Ambush camo for one of my 15mm scifi armies (Chuhuac space raptors). It looked garish, but still worked as camo.
I'm not sure if that was deliberate on GWs part, or if they just needed a faction colour for blood axes and purple contrasts really well with ork skin. Heck, they retconned orks having green blood (it's now a dark red) just because the studio team pointed out green blood on green skin looks crap.
