Ok, using two images I can get what I wanted,
So you can specifiy a 'team-colour' image and/or a 'rgba-image'.
You can use none, one, or both.
The team-colour one just multiplies the colour intensity at every pixel with the team colour, idealy a two channel image (grey, alpha) would do here, but the engine wont read 2 channel images at the moment, so use 4 and confine yourself to greys.
The rgba one is just a straight image, if you use both, the team-colour image is read first and then the rgba one is blended over the top.