I too have had this issue. I've got a 60 Gal barrel with lambic in it and I pulled some out after a year to see a crystal clear beer and the most pale yellow of anything I've made. to move that much beer, some goes into kegs and some into glass and some into buckets, all cleaned by starsan or 1 step (several people involved).
out of all the beer that was siphoned out, everything that went in glass or plastic was fine, stayed bright yellow and clear. on the other hand, EVERY portion that went into a stainless keg turned a royal purple hue in about 2 days time...some went into kegs just until bottled, some served from the kegs, but all of it was purple. not much difference in flavor or clarity either, maybe the purple version is a little dulled down.
This has happened with equipment from several people that has been taken care of in different ways, the only common thread is that the vessel that turned the beer purple was a stainless keg 100% of the time. What could be happening with this highly acidic beer going into a stainless keg that is making them change colors and loose some of the complexity of the flavors where the same beer going into glass or plastic sees no change?? I can't figure it out for the life of me....
as a side note: I saw this one other time with a tripel I made and stored refrigerated in a keg for about 5 months for aging(waiting for someone's wedding)...totally different beer and yeasts, I guess here the common thread is time. never seen this in other beers that have been in kegs aging for long periods (barleywine for over a year, other strong Belgians for many months)?
anyone ever come up with other possible causes??