We had a problem with the fairy lights. It was solved by replacing the batteries. The one on the left below had batteries replaced and the one on the right didn't.
With fading batteries, no matter which colour you press on the remote, you get red.
I'm not sure how they are wired inside but my guess is separate LEDs. Red LEDs have the lowest threshold voltage and would be the last to be still working as the output pd of the battery faded. This is connected to the photon energy hf - so colours with higher frequency have higher energy photons and need a higher voltage to get that energy. Full theory is here.
