Does anybody knows what is the technology the DUPLO Action Bricks are based on?It is not any RFID or NFC because I printed out the white one on normal white paper which turns lights on/off. The symbol on brick is not important - even white paper with symbol of printed rain drop turns the light on/off. I'm not able to make coloured ones - blue, red, green or yellow. Is the exact color shade so important? If so, then how to make own Action Bricks?
As seen in this question there is a sensor below the train. This is most likely to be a color sensor which is how it detects action brick.