Bruscolini.

I usually don’t write multilingual posts, but as I had to search quite a lot on the Internet for this solution, I think it may be useful to translate this one.

To make the long story short (the italian one is quite long, indeed): How To Make Your HP Color LaserJet 2550 Print Also If It Says That There Is No More Toner:

1. Turn your printer off.
2. Push and hold the green button.
3. Turn the printer on.
4. Wait about 20 seconds. The 3 lower LEDs (orange, green, green) turn on one by one, then the first two turn off again. The lower green LED stays on.
5. Without releasing the green button, push the toner carousel rotation button. The 3 lower LEDs turn on.
6. Wait a couple of seconds, then release the green button.
7. Start pushing repeatedly, as fast as you can, the red button. The printer LEDs start to blink in sequence, then they settle.
8. Turn the printer off and on again.
9. After the boot sequence, if the printer is not in the Ready state, push the red button.

This way, the exausted or nearly exausted toner cartridges will be recognized as “non original replacements” (as you can see on the configuration page you can print by pushing the red and green button at the same time) and you can go on printing. Be warned: if you insist in using a really exausted toner cartridge or drum you’ll end up in a damaged or dirty printer.

Other people got the same result in slightly different ways; this thread is a useful source of additional information.