Cross-contamination seems to be a likely possibility. I bought a new photo magenta cartridge and the ink is definitely pink (especially when you look at a thin film of it on the inside of the cartridge). Facial tones are now much better.
I read all the related posts but didn't see a tutorial...
I'm using a Canon S800 printer with only Canon inks (no refills). I've noticed that my recent photos are printing with very bad facial colors (rather bluish - everyone looks sick). Other colors are off too but it's harder to tell unless you have a comparison photo or the actual item to compare...