I like to spray-prime with black, but either before or right after applying the primer, add a drop or two of superglue to the top and bottom to secure the branches in position.
Painting the trees is pretty easy, and depends on personal preference. I like to use Ceramcoat or similar acrylics for terrain projects. I use a very thick coating of dark brown for the trunk. Then Black Green for the base coat of the palm fronds.
A brighter green for the next coat on the leaves, and then a very bright green drybrushing to finish.
Next, a quick drybrush on the trunk of lighter shades of brown.
Now you are ready to glue them to a base. This is where the roots come in handy. I like to use five-minute epoxy for this.
And here you see some finished trees. Two are being planted on this still-in-process DBA camp. The process is reasonably scalable, and, of course, no two trees will look exactly alike.