I haven't been able to use boat 4b 'If a tile has already been bid for, or it has been used for production, you don't have to follow the colour when placing workers on the tile' to bid for winter tiles with a different colour.
I've tried this in a couple of games and colours. Not trying to use green, not trying to use a mixture of colours. And it only happens when trying to bid on winter tiles, for example: 'Scribes' has 1 blue worker on it, cannot outbid with 2 red workers. As far as I am aware you should be able to use boat 4b with the winter tiles. This has happened on more than one game.
I've tried this in a couple of games and colours. Not trying to use green, not trying to use a mixture of colours. And it only happens when trying to bid on winter tiles, for example: 'Scribes' has 1 blue worker on it, cannot outbid with 2 red workers. As far as I am aware you should be able to use boat 4b with the winter tiles. This has happened on more than one game.