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I took this candle decoration apart. It cycles an LED through a series of colors so i expected a driver circuit, but discovered it contained only the LED & coin battery.
the switching mechanism was interesting in that it seemed to mechanically press the battery into the LED leads, rather than carry current itself.
instant rainbow, just add battery.
Hack-a-day has some more information (not by me)
The LED (R) resembled an ordinary monochrome, rather than a typical 3-color(L). A closer look reveals that the driving circuit has actually been built into the LED!