This uses an Atmel Attiny12 to drive a single 5mm, 4-lead RGB LED embedded in the doorbell switch (I drilled into it to remove the original incandescent bulb). Power is taken parasitically from the doorbell transformer (16 VAC, if I remember correctly) in the attic of the house somewhere, and the switch shorts the whole thing, ringing the bell and resetting the microcontroller. The color cycle starts with pure white upon reset, providing a visual indication the ring has occurred.
Just in time for Halloween! Nice work, Eiki!