The dark autumn evenings reminded me of a particular street light in my neighborhood that's an awful purple color. I finally dug into what causes that. The "white" LEDs are really blue LEDs with a phosphor-based coating on the lens. The coating is excited by the blue light and emits other wavelengths, approximating white light. If the phosphor coating degrades or delaminates then more blue light escapes, making the overall light look more blue-violet.
The post image is from an entry on the Dark Sky Consulting blog.
This recent research paper has some pictures of the actual failure on LEDs.
I used to install these for a living and let me tell ya: the failure rate of the first batch to bless the streets was staggering. I probably changed 1/3 of the first 1000 I installed because they turned GREEN. Understandably the public was in an uproar and demanded we switch back to high pressure sodium lights. We did not.