Why is the magenta color poor?

The only thing the internal procedures ever do is waste ink with no improvement 9 times out of 10, if it doesn’t make things worse. What you need to do is physically remove the printhead and soak it in water. As this is a later HP 900 series machine, the disassembly shown may not apply (only applies to HP 564/901-912 printers), but it is similar enough to make sense - just flip the grey tab holding the printhead up and it’s out. You may not be able to use the bypass on one of these, but it may not be needed in - as long as the printer recalls the printhead data. It’s included more for the 564/901-912 printers - they don’t have printhead recall, and losing the data wastes ink and forces it to run a alignment page every time. Buy a set of ink as a plan B since printhead prep wastes 1/4 of it on the example series. I suspect the 900 series OfficeJets are no better.

Refer to this guide to clean the heads manually.