I received a couple of pink LEDs in 2002 that did this. Ran for five hours at 18mA and the color change was VERY noticeable.
On the left is a brand new pink LED, on the right is its companion, after running for 5 hours. The LED was designed to be used in toys or other disposable articles that rarely see a second set of batteries.
Something with a short duty cycle, like a blinker or flasher, might also be appropriate.
And here is a "before" and "after" spectrum, showing the pink LED loses its ability to block green after it's been in use for 5 hours or less.
I believe pink LEDs from ETG Technology have a stable phosphor, and won't degrade like this.
The LED itself is as I suspected: a blue LED with an inorganic yellow glowing phosphor (typical formulation for an ordinary white LED); the pink color is from an
organic phosphor that is added to the formulation; this organic phosphor degrades rather quickly when exposed to the high flux found close to the surface of the LED die, and the end result of this degradation is you once again have a white LED; though with an ugly pinkish tinge at the edges. Run it long enough, and that may also pretty much vanish; though I doubt it will look quite like an LED that was made to be white to begin with.