The HPLIP panel and toolbox icons can be found at
/usr/share/hplip/data/images/, where there are folders for a variety of icon sizes. One may change the names of the stock icons by appending ".old" to each file name, then simply copy your desired icons into their respective folders (renaming each according to the original).
(Note that changing the stock icon names and otherwise editing within these folders will require root privileges. Use
gksu nautilus, for example, in a terminal; or
sudo your renaming and copying commands directly. Do be careful when making root changes within system folders.)
Some icon packs do contain suitable replacements (Captiva is pictured, https://github.com/captiva-project/captiva-icon-theme), but I'm not aware of a ready-made monochrome set. You may be able to create your own with Inkscape or Gimp.
I'm not aware of HP actively prohibiting home users from customizing the appearance of their Desktop icons. I think their team has been great to provide Linux support; I suspect they'd understand that you're not aiming to undermine their brand or flaunt violation of their policies by bringing aesthetic harmony to your PC.