The latest edition of “Inside Lighting Infoletter” web site  has an in depth article entitled” Investigating The Purple Light
Phenomenon In LED Street Lights”. Interesting conclusions.

Winnipeg got a call out and the pics look like the autobahn ATB0 that done turned blue in peg city
there was one on my route home I was watching slowly turn blue over a 1 year period and was 1/2 way transitioned when the supply failed and hydro in there usual have not done anything with it
last fall I happened to see it day burning and could look up at it running and see that the chips on the edge where blue with most of the ones in the centre of the assembly still white and it was an all or nothing on the chip part