If you look in the diagram on the create table page for table constraints, the only thing that can go inside of the parenthesis is "column-name". Since SQLite is so forgiving of quoting style it's saying "color" is a valid column name in the child table, so that must be what they're referring to.
If you had tried something like ... foreign key ('col', color) references enum... you would get an error along the lines of Error: unknown column "col" in foreign key definition.
The virtual generated column idea does sound like a good one though.