When the riso indicates that the ink cartridge is empty, the front panel can be opened to remove the cartridge (in the centre of the ink drum) by turning it counterclockwise so a new one can be inserted. The empty cartridge can simply be disposed of in plastic waste. Sometimes the device might indicate that the cartridge is out of colour even though it still has enough ink in it. In that case, there is probably just a bubble of air in the cartridge, preventing the device from drawing enough ink.
The cartridge can be removed by turning it to the left and, after attaching the screw cap, it should be shaken vigorously several times to thoroughly re-mix oil and colour in the ink. Then the screw cap can be removed and pressed lightly against the bottom of the ink container, making drops of colour leak out. This is best done with a long rod (e.g. wood or plastic) from the back through the large hole on the bottom of the cartridge lightly. Now the cartridge should be put back in again and, if necessary, some test prints should be made before proceeding with the print run.