Marc,
Wash the metal armatures in warm soapy water, dish soap will do, to get the mold release off. Dry and try not to touch them with your oily fingers. I use latex gloves.
Spray them with the cheapest grey primer you can get. I buy the cheap stuff at Wally World. Primer for anything will do.
Next, go outside and really look at a tree trunk. They are shades of grey, not brown, aren't they. I use "craft acrylic" paints and brush them on. I mix up a medium grey with a very little dark brown. You can dilute these paint for use in your air-brush and paint a whole bunch at once. Sometimes I use a little dark red oxide with the other two colors. Experiment and try to match the trees in your neighborhood.
When the paint is dry, spray dullcote to seal. Then you can go on to apply the foliage per the instructions.
And here's a tip. When the tree is finished, spray a light yellow color across the top of the tree. Not down on it, but across it just to highlight the top maybe 1/3rd of the tree. More paint should go into the air than on the tree itself. You want it to look like sunlight shining on the top. The underside of the foliage will look all the darker and add depth to the tree.
B-)
(I'm tempted to start charging for these tips. ;))
