All my experience has been with Tektronix (now owned by Xerox) 4C laser printers. If toner is running short you tend to get a bit of streaking or fading just in parts of the page.

If the image is light overall, that's probably not a shortage of toner or even the fact that you're using generics instead of factory carts.

I'd burrow into the manual and see what other "consumable" parts might be at fault. You've got components, for example, that "fix" the toner to the paper. Again, lacking any knowledge about an HP product, I would hesitate to tell you where to look -- except to see what the manual says or, lacking that, go to the company web site for info.

But this also could be something that's accidentally gotten reset in the menu.

The fact that the demo page is mostly okay is puzzling. Is the demo page solid bars of color or some actual photo or other image? 

Good luck with this.