The Yellow boat. The Red boat still has a way to go to enter its 3 length zone whereas the Yellow boat is very close, if not in its 3 length zone.
So Yellow is nearer to the zone. See definition 'Zone'. The definition refers to the area round the mark - not the mark itself.

I'm not sure it's that simple. The definition of Zone says "within a distance of three hull lengths of the boat nearer to it [i.e. the mark]". If the boats are exactly even then neither is nearer to the mark than the other. But that said it's probably impossible to judge or to prove that the boats were exactly even exactly at three boat lengths.

Does it matter though? They're still going to be overlapped no matter which zone size is used.

Very easily searchable is this same debate in various places including this forum before. Best guide is the Q&A 2016.001 answer on this topic available here https://www.racingrulesofsailing.org/cases/741/compare

Tim

The area around a mark within a distance of three hull lengths of the boat nearer to IT. A boat is in the zonewhen any part of her hull is in the zone.  
IT refers to Mark. IMHO
I think the definition says nearer to the mark. Nearer to the zone would be a chicken/ egg scenario. And I agree it would be impossible to judge when boats were exactly equidistant from the Mark. 

Paddy

Paddy, yes, "mark" is what I meant to say. Edited...

Here i go again.
Red is near the mark.
Yellow is not so close BUT it´s in its zone.
Which zone we must consider ?

Cheers !!!
Cata

If the boats are even when getting to the 3 yellow boat-lengths, then the zone is set there (both are the boat nearer from the mark). If red is slightly ahead of yellow the zone is set when red reaches his 3 boat-length from the mark (she is the boat near to the mark)

Again see this link here as it gives a good description of the likely scenarios. https://www.racingrulesofsailing.org/cases/741/compare

Perhaps the best way to think about it is when does 18.2(a) and 18.2(b) become relevant?
Some rules and zone definitions
Zone - The area around mark within a distance of three hull lengths of the boat nearer to it. A boat is in the zone when any part of her hull is in the zone.
18.1     - Rule 18 applies between boats when they are required to leave a mark on the same side and at least one of them is in the zone....
18.2(a) - when boats are overlapped, the outside boat shall give the inside boat mark room unless 18.2(b) applies
18.2(b) - If boats are overlapped when the first of them reaches the zone, the outside boat at the moment shall thereafter give the inside boat mark-room. If a boat is clear ahead when she reaches the zone, the boat clear astern at that moment shall thereafter give her mark-room.

So, firstly simplify the process, there is only one zone per mark, based on the three hull length of the nearest boat.  Boats do not have individual zone boundaries round the course, the zone is defined per incident depending on the boats involved. 
In your second example, red is nearer as you state, therefore it is her boat lengths that matter (see zone definition).  If yellow is inside boat when red reaches the zone as is likely about to happen then yellow is entitled to mark room because she is overlapped inside red when red first reaches the zone (18.2(b)).  As zone is based off the nearer boat and Rule 18 only turns on when the first boat reaches the zone (see 18.1), at the moment of your second diagram Rule 18 is not yet relevant as no boat has reached the zone, as defined by the boat nearest the mark i.e. Red.

If there are differences in boat speeds, then logically overlaps must develop at certain points. Thinking of it with boats moving may clarify the rules process for you rather than looking at one static image, this is why all cases etc. show the tracks of the boats.  
In your first example, as the boats progress, 
- If red moves forward compared to yellow, she becomes the nearer boat and therefore zone is on her hull length, does she break the overlap before she reaches zone?  If not then boats are overlapped at the zone and 18.2(b) applies.  
- If yellow moves forward compared to red,  she becomes the nearer boat and therefore zone is on her hull length and as she is inside boat is entitled to mark room. 
- If they continue at the same speed and remain joint-nearest (a highly unlikely scenario because of angles but let's go with it!) then logically they are going to reach the mark overlapped and therefore 18.2(a) broadly applies anyway (I believe!).  Additionally, they must have reached the smaller "option" for the zone at some point, so yellow has room as she is still overlapped inside, as she did at her own 3 boat length point so what is the issue?
A further question might be, what actions can red take to prevent having to give yellow mark room, this would depend on how the overlap was established in the first place and proper course etc. (Rule17)

Simple answer: The size, speed, and boat-length distance of the boat farther away from the mark at any point is irrelevant. It does not create a zone.

I think it's more accurate to say that there's one zone per pair of boats approaching a mark, defined by the length of the boat nearer the mark. Consider the addition of Blue to this scenario. Red and Blue are 30 foot keelboats, Yellow is a 60 foot keelboat.

Is 18 on:

18.2(b) applies with all 3 being overlapped per the definition of Clear Astern and Clear Ahead; Overlap

Sue
18 is turned on for yellow and blue. However a zone has not been established between yellow and red so 18 does not apply between yellow and red. Red is the closest to the mark between yellow and red so red determines the zone between them at this point. 

How about a variation where Yellow (the larger boat) is coming up quickly from astern of Red (the shorter boat) and establishes an overlap to leeward just before Red has entered the Zone defined by Red's hull length as the closer boat.  Now at what is the last second, from Red's point of view, Yellow has acquired rights to a seaman like rounding of the mark and Red will likely have to start changing course almost immediately to allow this.  Is this the correct interpretation?

From your comment assuming an leeward overlapped means port mark rounding and therefore Yellow has an inside overlap?

If so, then yes under 18.2(b) Yellow has mark room and rights to sail directly to the mark and Red should attempt to give her mark room. However, if (relative) speeds, positions and conditions mean that from the time the inside overlap from clear astern was established, red was unable to give mark room despite trying then 18.2(f) may apply and Red is not required to give it.

James Chapmen, that is exactly what I thought.  To me the late inside overlap by the longer boat is a good way to emphasize the way the rule works.  It is a perfect example of Rule 18.2(b)  applying to a snapshot of when the nearest boat enters the Zone defined by it's hull length.