Well, one reason it looks flat is because there are no shadows and our eyes like to see depth. I usually draw these in to give the model more definition, but I am on the fence because I would like to see what RW3 will do for me automatically.
For example here is another repaint I did which had some shadows added (credit to the person that did the repaint template).

here is another very small example:

vs.

Repainting the model has given me a lot to think about and learn because it started in a "blank" state by using the DDS raw:

As for the shade of brown... I had to struggle to get it this close for an unweathered look (see my older threads here at RWA for that). It will look a lot better when the dirt, wear, and rust get applied.