Depends

on the viewer perspective and on how you want things to look- a faded hazy vista or a semi-detailed extension of the scenery.  You can go with small artists' brushes for detailed tree tops up close, but I'd think you'd need maybe a 1" brush for slight blending/fading for a misty/distant look.

On my not-finished layout, I have a sky-only backdrop.  It is sky blue at the top and blended into almost white down towards the rails.  My layout height (44 1/2 inches from the floor) is such that you the viewer are in a river valley surrounded by Missouri bluffs.  As you look down on the layout you see most of the front of things, but also the tops of buildings and trees.  As you look at eye-level at the backdrop it will be the tops of the bluffs (yet to be built), with trees up there too.  I'm in a way trying to present the view of being down in a river valley and all you see is bluffs, trees and sky, not the stuff behind the bluffs.

No Green

You heard me - don't use pre-mixed green for any finished color.  Most such green doesn't look right next to ground foam or other scenery material.

For distant eastern hills, try varying mixtures of yellow ochre, ultramarine blue, and white.  For closer areas, try some kind of black (other than ivory black as it tends to be translucent more than most), mixed with cadmium yellow deep, cadmium yellow medium, and/or cadmium yellow pale, using the darker yellows for shadows and colors that are farther away.  To enhance some shadows, including shadow sides of hills, try mixing the resulting green with some acra red orange (red is the complement of green and will serve to gray out the green color).  And by "some" I mean rather tiny amounts.  For the lightest highlights add yellow light hansa.  Again you can add some white for certain effects.

The distant juniper forest is similar to paint as an eastern hardwood forest.  It's yellow ochre and ultramarine blue, with a bit of Liquitex Permanent Light Violet for the darks.  Darker colors go on first and lighter mixes build up to create highlights.  Creating your scenes should be simpler than mine as you won't have all the areas of yellow grass to contend with and can concentrate on greens.

The foreground trees here are entirely mixed using the black/yellow/red combinations I described.  Colors are built up in layers, darkest first and progressing toward lighter ones.  Don't expect a product that looks finished at any step until the last.  Also don't expect your first attempts to work out.  You'll get better with practice.

Pay attention to the lights

I like Rob's suggestion about mixing your own, and I suspect that it's one of those things which goes a long way toward improving the overall look of a layout - matching the foreground and background colors properly, that it.

One thing to be careful of, though.  Before you start matching colors, make sure you're working under exactly the same light conditions that you'll be using on your layout.  Different lights have VERY different color profiles, and what matches under one can look way off under another.

Trivial piece of knowledge

The observation of two colors that match under one light source but do not match under a different light source is called metamerism. Specifically in reflective surfaces (paint), the mismatch under different light sources is termed illuminant metameric failure. The mismatch results from the fact pigments have a wide spectral power distribution while your eyes ultimately reduce color perception to tristimulus signals (a combination of three colors).

Big color theory words to impress your friends and neighbors with.

I  used greens in the

  I used greens in the foreground and blue/ purple chalks over green to convey the Catskill Mountains in the distance.

Shades

It all depends on the viewpoint. If its the hill behind the track then its darker and more detailed, if its the yonder hill then it gets a small fade with the famous blue ridge haze.

Here is my first attempts at painting backdrop, dont ask what colors cause I mixed and changed, just made it look right to my eye with the room lighting.

Thank you all

Thanks to everyone for your information.  I bought some supplies and have painted a tree.....one tree.

  Good for you Rick ! Many

Good for you Rick ! Many modelers think about painting a backdrop but seem to never get around to it from fear of failure or other reasons. You have taken the plunge. One tree will grow into two. Watch em multiply as you gain experience in this seemingly scary undertaking. Keep us posted on you endeavors. 

I was always fascinated by the Bob Ross technique of painting background trees and always wondered how tough can it be. He seemed to make it look easy. Maybe Rob can chime in here. He's done some impressive backdrops.

Randy

Uh oh...

You never know until you try, fail, try again, and repeat.  People who make it look easy do so because they've practiced and made mistakes.  Rarely can anyone get a decent result on the first attempt.

That being said, I'm not a fan of applying Bob Ross techniques to backdrops, as I've noted before in other threads.  His ideas were to help the inexperienced have fun with painting, not necessarily to get results that look appropriate behind a model railroad.  He uses a distinctive style that jumps out as what it is instead of blending with 3-D scenes.  Maybe you could learn a few things from Bob, then get out the photos and enhance your skills reworking the shapes and colors for backdrop use.  With all the "un-learning" involved, you may be farther ahead taking instruction from somebody else who places more emphasis on realism.  There are books and videos out there for this, although I don't have any additional specific recommendations beyond the Mike Dannemann backdrop book from Kalmbach and the books on oil painting by Margaret Kessler that offer various good ideas.

Figure out some methods that work for you, and try them out on scrap material as you refine them.

Rob, Thanks for clarifying

Rob,

Thanks for clarifying that. What your saying makes a lot of sense.   As usual, more good advice from a seasoned pro.

Randy