Greg,
I will second what Bruce has stated above. Craft paints are great for indoor, non-weather related applications, but are not designed to stand up to the rigors of the out of doors. As for color matching, I had HD color match some paint to the colors of our club patch, and if it is different, my eyes sure cannot tell the difference.
Air brushing is a great way to paint, but for all the outdoor stuff I have painted, both hobby and non hobby, brush painting is as good as it gets. I pre-paint as much as I am able to do before assembly. I mask off the areas that will be glue surfaces to allow TiteBond III to get a good hold. When assembling I keep a supply of soft rags, dampened on hand to wipe off any excess glue that may squeeze out of joints. In most cases I also pin the joints with 22ga pins from a pin nailer. I am an air brush fan as well but sometimes it just isn’t worth the effort, plus most air brushes don’t have a large enough tip to allow that type of paint to be shot practically.
In short, outdoor railroading requires a slight shift in how things are done, more to the 1:1 world of process. You will find that your railroad as a whole will be a new learning experience as techniques that work in small scale don’t work well in large scale. Great looking projects, and a good looking railroad so far.
Bob C.