Just today I reinstalled slackware 9.1 on my laptop and then used cups,
http://localhost:631, and setup my HP842C.
In the Manage Printers section there is an option for Output Mode:
You have greyscale, Black & White, Color, and Raw CMYK
If you're using cups, you might check that. Or if you're using some kde or gnome print thing, then you might check in there that you don't have black and white or draft or something selected.
Also,
did you take the tape off of the ink cartridge head?
Just some thoughts...
Edit: oops, I see in your post that you
are using cups. Dang, and I don't like it when other people don't read
my threads more closely.
Sorry.