Getting a new HP printer to work via USB with a 2012 iMac running High Sierra 10.13.6

I'm having an issue getting a HP Envy Photo 7155 printer to work with an older iMac, running High Sierra 10.13.6. I hunted to find a printer that would work with this operating system and that had a USB port so it could be connected to her computer, rather than via WiFi. (She lives in an assisted living community with a shared WiFi that is not steady).

But --- when I hooked the printer up via USB 2.0 cable, it would only print in black and white (the print menu showed black and white, and there was no other choice. I called Apple Support, but they were not able to help. Ultimately, the support guy was stumped said to call HP support. I tried that, very unsuccessfully -- I could never connect to a real person.

So next try -- I connected the printer via WiFi and I chose to not allow the printer to be on the network. It worked beautifully -- colors printed, we had the normal default Mac print menu and we could change selections (including changing from color to black and white). Yipee! But, the printer keeps losing connection with the computer (probably due to the WiFi irregularities). The only way we can get it to work when this happens, is to restart the computer. And apparently it is happening quite often. : ( So, my dear mother (90 years old by the way and using the computer daily!) - is frequently having to restart her slow computer in order to print anything.

Has anyone had this problem? Is it because her iMac is so old? I just can't imagine why the WiFi connection works to print color, but the USB connection doesn't. Her computer, while old, seems to be working perfectly otherwise. I'd like to get it connected via USB - but she needs to be able to print color. There may have been other features unavailable on the print screen, as well -- I didn't fully check it out.

Sorry this is so long ... but I figure the more information the better the possible solution. Thank you for any ideas or solutions you might have!