Click on the Objects Tab in Terragen 2. Click on the Shaders Tab in the objects control/input. There should be a parts shader with a box to the right. This will bring up a new window. It should have an input with three icons to the right. Click on the one which looks like an upside down triangle; the node button.
In the node network window, below the preview, a list of the various Material/Shaders should appear. The red ones with the input connectors control color, images, and other aspects of each Material/Shader.
If I do not know where/what something is, I give it an obnoxious color, and do a small quick render. Now you are ready to color, and texture, add images as you need.
Looks like a really interesting render! I look forward to seeing the final image.