You are getting cross contamination which could be a few things:
1. The dampers are failing due to old age or just a bad seal somewhere and causing the cartridge to pull back on the system, which pulls the nearest neighbor's color into itself. If your dampers are over a year old, I would change them as they are considered maintenance items that need to be changed every 1 - 2 years to work as needed. Yes some people get more life out of their dampers but if you just change them as recommended by the manufacturer you never have to worry about these problems popping up in the middle of a big job.
2. If the yellow cartridge is empty but not flagging the machine properly, it will create suction that is similar to number 1 above. Just make sure there is ink in there. If there isn't, you just need to get a new one.
3. The head is being contaminated by some outside source such as ink build up on the spitting sponge, a wiper that is stuck in the out position or ink build up on the maintenance station and head surface. Make sure absolutely everything that touches the head has no ink build up on it and the head itself doesn't have ink build up on the bottom. This is common in 1204s because it's hard to get to the bottom of the head.
4. The worst case scenario is that the head is damaged internally and the cross-contamination is being caused by a broken channel. In that case, you need a new head. You can test this out as a last resort by trying to flush cleaning solution through the yellow and cyan channels. If you push cleaning solution through 1 channel and it comes out of another one, it's dead.