Colors will depend on your planned season. Lighter greens for spring. darkening and on to foliage colors for fall.
You should get a mix of fine, coarse, clump, etc, nature is not all one size. I started with a coat of tan paint (only do a sq foot or two), then sprinkled on some fine and coarse ground foam in a couple of different shades of green (I’m doing early fall). Moved on to the next section and did the same. Don’t sprinkle the foam on so that it looks like a lawn, have it thinner in some spots. You can get quite a lot done in a little time, just make sure to do a smaller area at a time so that the foam sticks well to the wet paint.
Once the paint is dried, follow the instructions in a good scenery book, putting down a glue solution, more variety of foam and a wetting agent. It will take several times over the same area to complete the job. You can add larger clumps of foliage to represent bushes. I usually do it with full strength white glue, some folks use tacky glue.
To answer your question on what to buy, the way I would do it, would be to have a large container of your basic ground foam color, with smaller packages of other colors and textures. If you are planning a large layout you may want larger packages.
For trees I wet whatever I have chosen for an armature, spray or soak it with an adhesive (see your scenery book) and sprinkle on fine ground foam. Some times I use a coarser foam. Don’t use all the same color on all of them, but use close colors.
For some trees I have used poly fiber stretched over an armature. Spray the fiber with adhesive and sprinkle on foam.
Background trees, I make puff ball trees. If your book doesn’t tell you how to do them, there is an article under “how to” (I think) above that will show you. After making a bunch of balls from poly fiber, I soak them in a 50:50 matte medium and water solution, then&n