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.
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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!
instant rainbow, just add battery.
Hack-a-day has some more information (not by me)
https://hackaday.com/2021/02/27/sounding-the-humble-led/