How does the DUPLO Action Bricks work? 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?
It is about color and length 

The Duplo action brick uses a color sensor, so you want to use the correct colors
and it needs the length of the brick for the sensor to determine the color.

paper replacement (youtube)

How to 3d print your own