To my knowledge there isn’t a way to do that yet… Check out Metabase’s GitHub Issues, this one is closed but may have been merged or lead to others like it. Search that forum and this one on obvious key words and not so obvious ones like “hex-color” or just “hex” too:
Pinmap with colored pins and clustring (GitHub Issue #2737)
Sign-in to GitHub and you can “upvote” up and down the threads of any open issue or open a new one, that helps the Metabase team know what is most important to users. And you can also do the same with “like” in this forum as they look at that too.
I had thought of including color in my own recent Issue because it wouldn’t be such a stretch to associate a key-field for a case like your own saying this bunch of banks are coded 1 and that key associates with hex-color #00FF00 (green), this other bunch are coded 2 and that key associates with #FF0000 (red), etc. But my post was already a bit involved:
Give Users Map & Legend Binning Control (GitHub Issue #9208)
Plus I’ve already annoyed everyone enough pointing out a particularly painful long-standing known issue!
Embedded GeoJson Maps Not Resolving (this forum)