They grow more quickly at higher temperatures and the fins look blue until the yellow pigment develops which happens after the metallic reflective pigments form.
That being said, the blue and yellow forms (except where they aren't) are found together in the same place at the same time. While there are only-blue (not many!) and only yellow (lots) gardneri they're part of what's normal/nominal for populations that are like this. Plus in the case of aquarium strain member of the gardneri superspecies the genetics can be so mixed up you can get anything: blue, yellow, even orange.
So it's possible but unlikely to have all blue in one and all yellow in the other but odds are they'll all look like their parents in 6 weeks or less. These are fast growing fish,one of the reasons some killies are not inexpensive is they take over a year to sex out. Celiae and cameronense come to mind. You could have raised four seven generations N. furzeri in that time.
These are some examples of blue gardneri; they are most often found with yellow forms in the same ditch at the same time:
F. gardneri lacustre "CI 1989-5"
This fish if very similar to the "German purple gardneri that were around briefly in the late 1980s. The story goes that a soccer team from Cameroon had brought over some fish to sell for some pocket money in Germany and unfortunately there was no known location associated with the fish. I received a pair from Germany i the simmer of 1989 and they were indeed purple and blue not green and blue which is more typical for GAR.
