Every airliner cockpit has them: square buttons that light up from inside, with the label as part of the light. FUEL PUMP in white, FAULT underneath turning amber when something goes wrong. Pilots call them Korry buttons, and a panel of them tells you the state of a whole system at a glance. I wanted a row of them for my flight-sim cockpit, and just as much for my workshop, where every machine panel ends up with buttons that only know "pressed" and "not pressed".
Real ones are big, expensive and made for aircraft. Cheap lookalikes are a switch with a lamp, and each needs its own bundle of wires. So the brief was: fingertip-sized, smart enough to look after itself, and chainable, with one cable in and one out, however many buttons are on the line.
That means a tiny CH32V003 microcontroller on every button, which costs about as much as the LEDs next to it. Add two RGB LEDs and a switch on an 18 × 18 mm board, and that's KaroKorry.