It has been sugested here to paint a flat latex earth tone over our “Plywood Central” or "Foam Flats Railroads. Thats great but it get’s boreing real quick, done that been there. I will post three pics to show the difference. The first is a flat latex $2.00 gal mismatch from a paint store. It looked a lot better and I happened to have a few of those Wal Mart 44 cent acrylic paints in various earth tones, thought what the heck, it aint gona get any worse. Using a 1 inch stiff brush, a tiny bit of paint, on a very wet brush I started tinting over the latex with very thin washes. the white in the picture is spray on cieling popcorn texture, had a can laying around and tried it. 
Using brown and burnt sienna, black, yellows close to raw sienna and yellow oxide, tans and hunter green, one can just about get any color of earth tone he wants. The important thing is very thin washes (little paint lots of water)
In this picture you can see the grey undertones for shadows, green tones around grass area, lighter and darker tones. These tones are the last wash, it may have a few other washes under them to bring it out in the last stage. I didnt put a lot of scenery in this area as I may tear it out and put in a curved bridge later.
Closer to the oak tree and you can see the various tints. An artist can plainly see I am not an artist of any merit, but it looks better then just flat latex, to my eye at least, Anyhow it a cheep fix and easy to correct if ya dont like it. 
The earth tones in this pic is a bit different, using gesso as a base, and thin washes of artist acrylics to get the earth tones. The main point is. Many washes overlaping and on top of other tints to get the final result. The la
Good tutorial. I can only underline the two main points
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lots of color variety. This is what DPB calls leopard spotting
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Lots of separate washes with the black last. I sometimes add some dry brush white last for sunlight.
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I might add that practice is as important as good information.
jwar, thanks for the good summary
Neat! Thanks for sharing the hints. Now that you have the technique down, if you took some color pictures, then looked at them in your train-room lighting, you could more closely match mother natures variations. This hobby is all about sharing knowledge and ideas, eh? Great work, jwar! jc5729 John Colley, Port Townsend, WA