Canon i560 inkjet only printing black - no colors.

My Canon i560 inkjet was working fine until yesterday. The warning
light started flashing amber, and the printer stopped printing. After
switching off and on a few times, it started printing again, but only
in black. There is plenty of colored ink in all three color ink tanks.
No amount of head cleaning makes any difference; there is not even the
slightest bit of colored ink coming through.

If I set the printer to pring in gray scales, it prints the colored
portions of the job in gray scales correctly.

I tried reinstalling the driver but it made no difference.

Can anyone suggest what might be wrong?

Thank you,

Al D