I assume you have this model:
https://everymac.com/systems/apple/powerbook_g4/specs/powerbook_g4_1.5_17.htmlWhat I found with the Poerbook6,8 is I had to use one of ELN's DEBUG Mac OS ROM files from the Mac Mini thread. I never did get things working very well on that model before I sold it on.
If you are just getting a gray screen, with no "Happy Face Mac" then you likely are never getting pasted the Mac OS ROM file, so you need to enable debugging in Open Firmware and give us a snapshot of the screen.
Command+OPT+O+F at boot, then:
dev / 13FFF encode-int " AAPL,debug" property
Then:
boot cd:,\\:tbxi
Or the Hard Drive path to the :tbxi resource( Mac OS ROM file ).
boot hd:10,\\:tbxi
That assumes the Mac OS ROM file is in a blessed system folder on the 10th partition of your HD.
You may also just boot the :tbxi without a System Folder by providing the absolute path.
boot hd:10,\debug\:tbxi
That assumes you have the Mac OS ROM file in a folder named "debug" on the 10th partition of your HD.
You can put the Mac OS ROM file anywhere, when it goes looking for a blessed System Folder, if you have on on CD/HD it will find it.
To get a directory of files and folders on your HD from Open Firmware:
dir hd:10,\
Or
dir hd:9,\
And so on by partition number.....