In short, no; there are a couple of threads about 1/300 'fleet scale' battles; and a few threads with 28mm 'gunboat' nefs in them, and they're different depending on the modeller (some don't have tailfin insignia at all as far as I can see). One modeller (not in the Atlantis campaign I don't think) painted a Union Flag on the tailfin. Several fly the white ensign from rigging, but have something else on the tailfin.
As for nefs specifically serving in the Atlantis campaign (so in service between 1885 and 1900), in
this thread the nefs are painted blue-white-red on the tailfin (reading front to back); in
this thread, they're painted red-white-blue.
Apparently the real RAF didn't settle which way they painted them until shortly before WWII (red-white-blue). This on one occassion led me to accuse someone of painting his nefs French instead of British, because he'd painted them in WWI blue-white-red. I'd like to apologise once again for suggesting such a terrible thing!
My advice would be to follow the most common convention for the small nefs if that's what you're doing. Perhaps, and this is a bit
post hoc, the Army (or the Royal Aeronautical Service, or whoever) paints their small nefs blue-white-red like the Balloon Lancers in the massed Atlantean battle, and the Royal Aetheric Navy (or whatever branch of the service it is) paints their massive capital ship nefs red-white-blue. Only a suggestion, but it might just work.