The coloured lights need some sort of power source, which usually comes in the form of a lithium battery at the bottom of the candle (separated from the wax so the battery won’t burn). Connected to this battery inside the candle are LEDs, most commonly three, which give off the three primary colours of light – red, blue and green. Different combinations of brightness in these LEDs causes the light to mix which yields many different colours.
So how do the coloured LEDs switch on when the flame is lit? An optical sensor runs from the top to the bottom of the wick in the candle and is connected to the LEDs and battery at the bottom. As soon as the wick is lit, the optical sensor detects light from the flame, which activates the circuit.
Author: Rik Sargent, Science Museum