AFAIK you cannot prevent the rasterization of the images - if for no other reason than that's what the colour one already is (it's in PNG format). However, if you configure Word to not compress pictures in the document (File|Options|Advanced>Image Size & Quality) and, when you save to PDF, click on Tools>Compress Pictures>Use document resolution, that should maximize the image quality.

FWIW, when I print to PDF using Microsoft's built-in PDF printer in Windows 10, the output is of far higher quality than your save_as pdf. Of course, you don't get any bookmarks or links when you do that...

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]