Catalan
Yellow has not reached the Zone. The Zone is the grey area in your diagram and a boat only reaches the Zone when any part of her hull is within that grey area. 
So Green reaches the Zone first and has an inside overlap and is entitled to Mark Room. 

Rule 18 applies as one of the boats is in the zone and none of the exceptions are true.

Rule 18.2(a) could apply as the boats are overlapped except that 18.2(b) applies.
Rule 18.2(b) applies as the boats are overlapped when the first boat reaches the zone (the first 'if').

So at the time of the diagram, Green is entitled to mark-room.

There is no definition of "FIRST" in the RRS, so the practise is to take the common definition of the word.    I guess there might be some ambiguity about if it is FIRST to reach the zone or FIRST in the race, but in this case I don't think there is any difference between those two.   In your diagram, Yellow is indeed further south than Green, but that is not the course of the race.  The course is to sail to the mark and round it, so Green is closer to that make and thus further along the course than Yellow.   Green is both first in the race and the first to reach the zone.

Note also that the definition of Zone is based on the boat closest to the mark, so that ties in well with the interpretation of the first boat to reach the zone.

Greg
The Green boat was the first of the two boats to reach the Zone. 
RRS 18 2 (b) qualifies the word “first” by “ of them reaches the Zone”
I can’t see any confusion or ambiguity.