The "DR-241" are the drum units into which the toner cartridge can be inserted. The toner has the designation "TN-241". If you simply need "new colour", you must therefore use toner (TN-241) and not drum units. If you only want to replace individual colours, you can also buy them separately. For black, for example, that would be here:
Brother TN-241BK (Toner, BK)