Since Seraphon are supposed to be hanging out in ships above the mortal realms, how do you justify lore-wise choosing a realm to take artifacts from or theme your paintscheme and basing? I'm thinking that maybe your particular Slann watches over a specific realm most of the time? They got around this idea for Stormcast by saying that the different stormhosts are now garrisoned throughout the realms (for example, many Hammers of Sigmar are stationed in Hammerhall Aqshy). Can you imagine something like that for Seraphon? Maybe a skink priest has a base of operations on the ground somewhere? Thoughts?
I would probably justify it along the lines of “The Fangs of Sotek have been engaged in a long campaign in the realm of Chamon, fighting against various forces of the Dark Gods and recovering many of the relics found there that were at risk of being despoiled or misused by Chaos”.
What's great about AoS fluff for the Seraphon is that you can basically do whatever you like fluffwise and it can still fit the official lore. Since the Slann are more scattered and independent, while still being all knowing space frogs, they can basically pursue their whims on how best to fight chaos. So if you want realm based Seraphon no problem, your Slann got stir crazy sitting on a ship for eons so he's built himself a life on his favorite realm!
Tying your minis (and perhaps terrain collection) to a specific realm is a good way to add fluff to your color scheme.
I remember a brief commentary about seraphon going native after spending a prolonged period of time in any of the Eight Realms (can't remember the source). They still retain their celestial heritage and I'd presume could last indefinitely should the Slann that created them allow it, but over the course of their existence they become attuned to that realm.
Another alternative is that the Slann took on the essence of one of the Realms and this spilled over to the Seraphon he or she creates with his thoughts.