It is normal for there to be ink on the outside of the cart and nipple leading into the printer. This will build up over time and is nothing to worry about, unless it’s running down the wall into the cartridge bays, then I would be concerned with leaking seals in the cartridges. This should have no affect on black or cyan ink in the damper end of the ink supply.
Normally, when you see back suction of ink up into the damper of a different color is when the capping station is really old or dirty and gummed up with pigment. The print head will rest on the gummed up capping station and then suck up what ever color is usually next to each other. You said you replaced the dampers after you replaced the capping station, correct? If this is the case then this scenario should have been eliminated completely. UNLESS, the print head already had some of that ink inside the yellow channel from before, which I would have suspected to have been moved out when you performed the cleaning cycles after replacement.
Try the new Yellow cartridge we are sending you, make sure it has a good prime on it before installing it. Once installed I suggest running 2 cleaning cycles, the a QTR flush image of the Yellow channel to verify the repair. If this doesn’t solve it, then the next thing would be to replace the Yellow damper again and remove the print head to give it a good thorough flushing with the head cleaning attachment. At the same time, clean the Capping station and Wiper blade again and make sure the Flushing box is cleaned and flowing freely.
After all of that, if the problem still exists, the only possibility left would be the print head manifold having a crack in the very thin film inside, allowing the cyan or black to enter the yellow channel. But, if that were the case, I would more think it would be the magenta mixing with the yellow because they are side by side.