People have refilled the used cartridges, from what I've read. The thing is that the washer will pull detergent from both cartridges, regardless of the "whites" or "colors' setting. Depending on the setting, a certain percentage of Phase1 and Phase2 is drawn into the drum. If you refill it with regular detergent, I would imagine you'd have to experiment a lot to get the washer to dose the right amount (since both cartridges are activated).
Hendrik, do you have any idea how to get the correct dosage without being able to deactivate one cartridge, like it's possible in... everywhere but the US?