How to set the thumbnail or "main" image of a stack?

Hello!

I'm a new Immich user, and it's been working great so far!

I have images that come in color, black and white, and sepia versions, and I've stacked them together. How can I set the thumbnail and "main" image?
It would be nice to always show the color version.

AI assistance wasn't helpful, as the models don't seem to recognize that stacking is an available feature.

Thanks so much!