Scanned about 20 photos yesterday and eventually the scanner started scanning partially in greyscale. Certain areas of the image will be in colour and the rest will be black and white. People will have blotches of colour on otherwise grey faces. I have no idea whats causing this and haven't been able to find any solutions across HP's site.
Welcome to the HP Support Community. I'd be happy to assist you with the scanning issue.
To better assist you - Are you experiencing this issue while printing or making a photocopy as well?
Let's try these steps to reset the printer -
1) Take out the cartridges.
2) Unplug the power cord from the printer & wall.
3) Wait for 30 seconds.
4) Plug the power cord back into the printer and wall, ensure the printer is plugged into the wall & not to a surge protector.
5) Insert the cartridges back into the printer.
Download and install the printer's firmware from this link for macOS 10.12.