I talk to myself out loud; now I'm doing it in a forum.
Just found the answer: There were 2 photo carts, magenta and cyan. Replaced those with regular carts (1 Canon, 1 Inkstop) and all is fine.
Everything except black text has a yellow-green tint when I print color with my MP780. Grayscale printing is fine.
I soaked the printhead in Windex, cleaned the purge pads (but didn't clean the purge lines with fishing line), aligned, head refreshed, and cleaned. Nothing helped...