The situation is complicated, and I don't have a definitive answer.
Of the 15 Fearless Freddie's narrow-gauge coaches in my possession, two are the 2006 vintage described above, and these have the brightest green paint. One of these is no.5 in the photo below.
Note that I'm saying these are all Freddie's coaches. I know the Plarail version of Mighty Mac came with one like this, but these examples were all obtained in the UK where Mac just came with the
motorized coach and a grey truck, whereas Freddie came with two of these.
The remaining 2005 vintage coaches are predominantly a uniform bluish-green like no.4, but there is some variation in the bluishness.
Three of them are different:
No.2 is a very bluish-green, you might well call it blue, and it's the same colour on all four sides.
No.1 is the one from the auction lot that started this thread. On one side and one end it is bright blue, but on the other side and the other end it is slightly more greenish than no.2, though still more blue than green to my eyes.
No.3 is a pretty standard bluish-green on three sides, perhaps slightly bluer than standard, but on one end it's as blue as no.2.
It has to be remembered that this is a different situation compared to the greening of Gordon, etc. when exposed to sunlight or smoke or whatever. In that case we're talking about a change in the colour of the dyes in the plastic. In this case we're looking at differences in the colour of the paint.
If the paint was changing from green to blue as a result of prolonged exposure to strong sunlight, we'd expect to see marked yellowing of the unpainted plastic, but there is no evidence of yellowing in these examples.
My gut feeling is that we're looking at examples of poor quality control in the paint-shop. Using a strong magnifying lens I think I can see places where there's a bit of overlap between different colours on the corners of the ones that show more than one colour. IF the painting process involved several stages where different sides and ends were painted separately, interspersed with drying intervals, and IF there were some problems in the paint colour mixing department, then these effects could be achieved.
Bottom line - I really don't know.