Betta gender

[Flumpweesel]

So I bought a pair of females but now I'm not so sure that red here is girl, it's much bigger than the other and it's fins are certainly growing .

So what do we think did the juvenile get dropped into the wrong tank at the shop

If better angles are required for pic's let me know

[Anon]

Looks female to me. Males will have long ventral fins. The fish in your picture seems to have tiny ones. 

[AndreaW]

I agree it looks female. The organs look like they sit lower in her belly, and if you look closely on the under-side, you might see an egg spot (hard to see most of the time though). Male organs sit higher, more along the center-line of their body.