Edit the on-screen appearance of Contacts in Tahoe

Column widths can be adjusted. Hold the mouse pointer in between them, the cursor changes to arrows and you can drag the border.

Background color cannot be set by the user at this point. Only Apple can change that.

To have Apple consider your feedback, submit it here:

Feedback - macOS - Apple

FWIW, if you set a "Contact Photo & Poster" it gets rid of the purple background.

Note that it replaces the purple background with a background based on the contact photo. In your example, it's dark gray. Try a different contact photo, for example:

I do say so.

The algorithm is not taking an average of the hues in the contact photo, it's attempting to identify the background color in the contact photo and then sets the contact card background based on that. The background in your second example is white, so you get a dark gray background.

You just need associate with people/companies that have more style and don't use black, white or gray as their background colors. Like my friend, Julian.

At least one other person (on this related thread) sees what I see, i.e., setting the contact photo to white gives a white background for the contact card). I suspect that's the default/intended behavior, it's analogous to what Apple does with Music in CarPlay, where the background is a strong blur of the album art.

If you say so.

Thankfully that doesn't happen with my Macs.

It appears in neuroanatomist's and my Macs. Don't know why it doesn't in yours. And yes, it applies to individual contacts. There's no global setting.

FWIW, no black, gray or white…

Evidently theres's something different about our setups...

I tried playing with Appearance settings, they don't make a difference for me.