The motor controlling the belt movement will be controlled by a microcontroller. The micro-controller reads the color information or the card number incase a RFID is used and controls a bifurcation mechanics which will be either a moving robot arm for pushing the object to a tray or doing a pick and place operation.
The color can be sensed using a RGB color sensor like TCS230 which provides a pulse width modulated square wave output proportional to the Red/Green/Blue color intensity detected. The number of Red,Green,Blue bags detected can be displayed on a LCD display for information.
You can also implement a heat sealing mechanism for sealing polythene bags at the belt end.
Click on this link to know more about how TCS230 RGB color sensor works!