Okay, so I’m only painting to good wargames standard, but you can achieve some incredible things with Contrast Paints. I was, for example, very pleased with my ECW command figures (Essex 15mm).
So you now have the right figures painted to a good approximation of the existing element: all you have to do next is base them.
This is where I came more of a cropper. Firstly, for my existing army I had used thin metal bases, but had now run out. I could have sourced some, I suppose, but in the end decided to take the easy option and use my now-standard Warbases premium mdf versions.
I also had no paint that matched the existing bases (a very oddly light green) and had no note of where I had got in from. Worse, I had totally run out of the flock that I had used previously and, again, had no idea of where I had got it from. In the end, I bit the bullet and just went for the closest thing I could find in my flock collection: it doesn’t match, but it’s not too bad.
So there’s what you have to do to match old and new units:
the figures
the colours
the painting style
the bases
finishing the bases
I’m pretty happy with the match, and I’m sure that under “battle conditions” I’ll stop noticing any differences within minutes.
Post Script
I also painted up a unit of what Museum call Brigandines and I’m using as Billmen: