Can a LED change colour over time?
I've had a blue LED set up for some time and it use to be a much brighter (ultramarine blue) now it's heading to a more greenish dull colour. How is this happening?
My guess is over-voltage or over-current, both of which can be caused by using an incorrect resistor with the LED, or by a power supply giving more voltage than you thought. Slightly overdriving a LED will often work at first, but it will wear it out much sooner than if the specs were followed.
Sadly, not all LEDs are well made, and some will deteriorate over time even without mishandling. How long have you had this LED running? If it's just a few hundred hours, it probably was damaged by something, but if it has been in use for thousands of hours, it may just be aging.