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.
