That's old stagnant bile and maybe what very little gets produced when you are not eating any fats like on the MC. When you fast and are not eating fats bile production is almost nil so you are not going to buid up bile.
My first couple of MCs before I knew any better I ended them with and LF on the last day. First I consider myself lucky not to have had a bad reaction after dumping a buncha oil in my system after a long fast and second my other LFs after waiting to be back on solids and ingesting fats were much more productive.
If you think about it just simply makes sense not to shock the system after fasting by dumping an innordinate amount of oil into it all at once.