My daugther also has a Pix780 so I just replaced my print head with hers and now the colour prints great. I guess I'll try cleaning mine first and if that doesn't work, I'll purchase a new one when the dealer who services Canon opens on Tuesday. I could also buy one from the Canon website for something like $60. I copied the following from Canon's sites under the FAQs. Sounds like you may need to replace your printhead.
How does a Canon Printhead work?
The printhead delivers very small drops of ink onto the paper. The ink from the cartridge is transported via small tubes. Heating elements heat the tubes, creating a small bubble of ink. The heating element determines the size of the drop to be applied to the paper.
How do I know I may need to replace my printhead?
There are a few symptoms that may indicate printhead failure or a clogged printhead. If the following symptoms continue even after replacing the ink cartridges and trying the troubleshooting steps in your user's manual or those listed below, you may need to replace your printhead:
Why would the printhead have problems?
- Pages come out blank, or you see poor print quality
- Pages you print appear to be missing colors
- Colors on the printed page are quite a bit different from what shows on the screen
- Horizontal white lines or light-colored lines or light streaks appear throughout the page
After hours of printing, the heating elements may fail after being turned on and off hundreds of thousands of times. The printhead can also become clogged or blocked with paper debris or solidified ink. If your print quality is poor or your printer will not print at all, it may be time to change your printhead.