If it was sold to you as a green severum there's no reason to think otherwise because of the fins. The orange to red fins are a common feature of the standard greens and some will also get a bit on the chest and throat as they mature. If you were thinking maybe a rotkeil, not likely someone would sell a rotkeil as a standard green. Since they've been in the hobby quite a long time, green severum breeding strains vary some in quality and color. Wild strains vary also, since they inhabit a wide range.
If, by chance, it was a rotkeil you probably wouldn't see it at that size, yet, but it would ultimately be very apparent, not much mistaking the two once the rotkeils start to color up. The fact it's growing fast also indicates green to me. Won't say I know it to be a hard rule, but both in my own experience and from what I've read, rotkeils tend to grow slower and often don't quite reach the size of greens, golds, etc.
Nice looking tank!
