All cartridges must indeed be inserted and are also slowly used up, even if you only print black. The printer slowly sucks the cartridges empty sometimes after switching on, sometimes after switching off, before and after printing, probably to "clean" the print head. But luckily there are cheap compatible cartridges :-) Otherwise, I agree with what Detlefs40 said. I really like using this printer. I've had it calibrated and it prints great photos on compatible (cheap) paper up to A3+. I have several photo projects and so far all customers have been satisfied.

In my experience, all cartridges must be inserted and must not be shown as empty, otherwise the printer will refuse to print any more pages. (Print quality)
Printing "only" black, however, can be easily configured in the print settings.
Yes, black is really opaque. But it can vary depending on the paper. (Even with standard printer paper). Surface is crucial. (smooth / rougher / glossy)