Every colour is printed with a separate colour drum. To switch them, open the front panel and the drum should unlock automatically. If it does not, it can be unlocked manually with the green knob on the right under the drum or in the centre between the drums (depending on whether it is a single or dual colour machine). 1
The best way handling the colour drums is grabbing the green handles at the front and back, then pulling them forward on the metal frame and lifting them out of the anchoring. When carrying the open drum, it is important that it is handled carefully to avoid damaging the screen of the drum.