Well...I came up with this a while ago and wasn't sure if it's too much of world-building, but I'll post it anyways.
You know how, in the Splatoon direct, they mentioned Inklings are able to change their eye color and skin tone, and how someone else inputted that they can change gender at will? Well that got me thinking. If
all Octarians can change genders, at least, then that means that what we see in Octo Valley in no way accounts to the whole race. You know why?
Because it's all a military standard.
Octolings can be male or female, can change skin tones and eye color, and can wear what they want outside of the military.
Inside the military, however, they all look the same, as per standards set by Octavio. This also applies to the other Octarians, although I feel like those Octarians have to be in the military and aren't represented in normal Octarian culture; Octarian breeding farms? Military schools? We may never know.
But, in conflict with
@ReedRGale 's headcanon about stone-age Octarians, I believe they are much more advanced than Inklings. By the looks of things, the Great Octoweapons are actually robots - which suggests biotechnology may be in use. Then there's the Suckerfish and the Octostamps, and the strangely high-technology Octogoggles that I assume are used to enhance aiming and sight for the elite Octarian forces (or just Octolings, if you will).