Wow, I'm liking the look of these jb!

I really can't fault you with how smooth you've got your green - I struggle all the time with my method

As for freehanding flames, practice just drawing flame shapes on a piece of paper. Once you get the general gist of how fire is shaped, transfer it onto the banner with a deep red basecoat. From there, on, just build the colour up from red, through orange, to yellow (with maybe a hint of white for a really hot flame - be warned though, it tends to make it look paradoxically 'cold'!) Having told you all that, I actually rather like the understated black banner. It says all it needs to - the descendants of the 18th Legion, it kinda has an air of mystique about it.
If I was to pick out a couple of points it would be:
A) Slight paint nicks - On the hammer arm's elbow joint on Vet. A, you've caught the top of the elbow guard. It's not major, but my eye is drawn to it. Similarly, around the Crux Terminatus on his belt, there's gold smears at the back. On Vet. B, the scrollwork on his shield has caught the drybrush of gold, and it's still green. I'm guessing these two Veterans are still
WIP, so I thought I'd just nitpick if you haven't noticed them already.
B)
IMHO, the eyes on the Vet's could use a little something. Maybe not as drastic as
OSL, but even just a Gem/Lense effect could work wonders.
Anyway, my pedantic nitpicks aside (I was just trying to help

), these are some really superb models. Excellent work, keep it up!