every block contains a RGB-LED, it will get a random color at the beginning.
Thinking through EEPROM (24LC256) options there may be a way to use an arduino for each leg. The connections would create a "bus" for that leg that would talk 2wire to the EEPROM. When the RGB values are set, so is the EEPROM value. When a new block is added, the arduino on that leg would read the EEPROM and if compare with the last added. In order to actually change the RGB color to flash or otherwise notify the user, that would be tougher. I would suggest with this implementation you go with a central buzzer or light as an indicator of incorrect color. Lots more to consider, specially the 4 or 8 EEPROM limit per arduino.