Red is the inside boat.  The easiest way to see this is to slide or translate the boats from where they are to them going around the mark.

I agree with John as they could not round rhe mark without changing tacks.

I probably should have said a little bit more.  Red being the inside boat is just the situation at this moment and is used in applying RRS 18 to the boats in this position.  When the boats tack, as they certainly must unless there is an incredible amount of current, the application of RRS 18 will change as the boat entitled to mark room (Red) will have passed head to wind.