Well, as far as I'm aware, they don't have a strict method in the liveries they choose, and it looks as if it is more decision per aircraft, when it gets sent down for paint. This is evident I suppose when you look at the 738's in the G-GDF* series.
G-GDFP, G-GDFR, G-GDFS are all in the standard Jet2 livery. The we have:
G-GDFU - in holidays livery,
G-GDFV - in red livery,
G-GDFW - in holidays livery,
G-GDFX - in red livery,
G-GDFY - in red livery,
G-GDFZ - in holidays livery,
and of course G-JZAH in the holidays livery also.
There is no system as to where the liveries are deployed, i.e. Jet2holidays aircraft don't only serve the sunny Spain and Canary Island routes. Both liveries are treated as one, and the cabin fit inside is no different, still all with the red and grey seats. So it means that an aircraft like G-GDFZ, could be on MAN-TFS-MAN one day, and then be off to BUD, or CDG the next, and the same applies for the aircraft like newly painted G-GDFS.
In terms of Allegiant, Jet2 have pretty good relations with them, and it seems Allegiant have no problem with it, as although the liveries look identical, there are certain brand guidelines which class them as not. Like the sun on the tail - on Jet2holidays aircraft, the points of the Sun are rounded, where as on the Allegiant aircraft, they are not. There are also other slight differences, the colours are slightly different, and the positioning of the sun on the winglet, (when appropriate) on Allegiant aircraft, it is on the bottom, and Jet2holidays, in the middle.
I can only agree that it is the success of the Jet2holidays brand that makes Jet2 paint the aircraft in both liveries - I suppose it makes them stand out a little for doing so too!
