So, apparently, Marlin has this new(ish) feature of using the extra low-power outputs on Azteeg X3 for temperature warning lights.
I've burnt my finger the other day while replacing a broken hotend - the heater was off, but the temperature was still way way up, turns out E3D hotends take quite a while to cool down, especially with ceramic insulation strips applied on the heater block - and decided enough is enough.
Enter - The Christmas Tree Hadron.
Measured temperature (any) > 55 turns the RED lighting on:
... while temperatures < 55 enable the BLUE lighting:
Blingy, but useful.