We cannot color by lookup fields. Even if “Allow linking to multiple records” is disabled on the Record Linker, the option to select Single Select Lookup fields is disabled in the record coloring menu. This makes no sense, and is a real hamper on operations that make use of Airtables Sync feature to integrate multiple bases with each other.
I understand that the option is probably disabled because lookup fields are arrays, but there is absolutely no reason it should be disabled when the linker field is restricted to only allow the linking of one item. This is an oversight that forces users to spend a huge amount of time drafting custom color rulesets.