You're being much too modest about your knowledge of c o l o r. When I see photos of your layout, I am drawn in by the c o l o r s. The first impression I had of your layout is that it looks so real
because of the c o l o r s. Are you telling me that you simply got lucky in your use of c o l o r?
Thanks, Gary. You're right, though: I think that I did get lucky with my use of colour, and that may have been because of Woodland Scenics' limited range of colours offered. I had some older ground foam from an unknown company that had some colours that I found useful, but there wasn't enough of it to be of much actual use. Most of these were more muted than the WS products, and there were a couple that had a definite blue cast to them, very useful for simulating distant foliage. I was left with what was available, and tried to make the best of it. I think that it's important to remember that trees, grass, weeds, even fields of crops made up of all the same variety of plant, are all multi-coloured.
....Which brings up another question which I hope I can get some commentary on:
To be considered realistic by the average person, should the layout be scenicked following what people perceive to be real or should it be scenicked following what is actually real? .
Your question points out part of the problem in achieving realism: we all
know that a tree is green, grass is green, those crops are green, etc. etc. However, because we
know, we tend to not look, and therefore not see, that all of these greens (the parts that actually are green) may be different greens. We also don't see that there are actually other colours mixed in with the various "true greens", and that these colours contribute to our overall perception of the specific green that they're in. I didn't go outside to look at a field or at trees, (although I probably should have) because I "know" what they look like. However, everyone else also "knows" what they look like, although their perception could be radically different from mine. Any of us could be correct or incorrect, and I was just lucky to be able to reproduce my perception and also lucky that you found it realistic. I'm not sure that everyone else would, though.
....
Let me give an example to clarify: I was driving on an old asphalt road yesterday. The road was not black or dark gray, it was actually a very bleached out tan color with light brown speckles of gravel in it. Now, if I was to model that color, most people would not be led to believe it was a very good example of an asphalt road. So as a modeler wanting a layout that is believable to the average visitor, one should model what is "expected" by the average person?
You're correct in assuming that we expect certain things to be a certain colour, but we may or may not accept another individual's interpretation of that colour. I just got back from a trip "up North", and some of the highways there are a definite pink colour (this could be a real boon for those foam-based layouts)

due to the local granite used in the making of the asphalt. If you've seen this, and someone showed you a layout set in that area, you'd probably accept it as realistic. For everyone else, I doubt it, although maybe if the modelled road was passing through a rock cut, it might look "right". The roads on my layout are too dark, and they're also unweathered: I was trying to save up enough projects on the layout that require the use of an airbrush to make the task of setting up the compressor, etc. worthwhile, plus it would help if my wife would leave for a couple of days so I could get those jobs done without complaints about the noise, smell, dust, blah, blah, blah.

To get back to your question: if you model the road that you saw, as you saw it, then other elements should be there to contribute to the realism, such as the colour of the gravel along the shoulder, the dirt that's visible, or even the colours of the plants and trees nearby. Your mind accepted that tan-coloured road at least in part because of the other elements around it. Your observation that colours appear more realistic on a layout if there's none that are too much in contrast with others illustrates this thought.
Another aspect of this discussion is tricking the eye into seeing what you want it to see. Any decent artist can fill his painting with sunlight so real that you can feel its warmth. We seem to be limited in this ability by the low light levels over our layouts, and how many of us would be willing to add bright white highlights to our trains or scenery, or have the shadowed sides of buildings painted dark blue or purple? I
was surprised, though, at how much "light" can be added to a tree simply by dusting it with an unaturally light green or yellow foam as a final step.
Another idea you could explore in the search for realism is placement of the scenic elements. I had most of the scenes on my layout pictured in my mind long before the benchwork was even built, although some had to be changed or discarded for practical considerations, and other new ones presented themselves as the layout developed. Try to lead the viewers eye to what you want them to see. Usually, in a model railroader's case, that's the trains. So the scene should offer a good place to just view the trains - nothing too "poke-you-in-the-eye-look-at-me" stuff, but a "setting". Bridges seem to be my favourites, although I also like urban and industrial backgrounds. For an "operator's" layout, the industries that are the reason for the "operation" can be a good setting for the trains. However, if you look at real-life settings, many don't offer an unobstructed view of the thing that catches your eye and makes you want to look. Don't be afraid to add a few of these "get-in-the-way" elements to your layout. I have lots of telegraph and power poles, lineside signs, trees, and buildings, etc., that help to put the trains in the midst of a scene. This has residual benefits besides the obvious ones, too. Often, you'll need to compose your photos more carefully: perhaps a pole hides an important element of the photo, like a loco number. Moving the loco just a bit can not only reveal the number, but also direct the viewer's eye to see what you want him to see. Another benefit of these elements is that they can be used to make the layout seem larger, and the trains longer than they really are. Things like mountains, forests, or buildings, with the trains running at least partially obscured behind them, helps to recreate the context of how we see real trains.
Another aid to realism is to know when to ignore the details. I have a lot of trouble with this one. For things in the background, keep them simple. Because our actual distances from foreground to backdrop is so foreshortened, it's easy to over-detail stuff that's meant to be miles away. In the pictures below, the stuff along the horizon was my first attempt at modelling distant trees. It was very hard to resist making small (and smaller and smaller) trees to fill in this area, but it would've looked even less convincing than what you see here. What I
did end up using was a strip of white cushion-type foam, painted (with a 2" brush) using the same dark grey-green latex house paint that I used for my "rivers", then loosely covered with polyfibre, and sprinkled with some ground foam. This would've been an area where that "bluish" foam that I spoke of earlier would have been especially useful. Because it's not detailed, it doesn't attract the eye too much, or at least I hope that it doesn't. Of course, I just spoiled that by mentioning it, I suppose!
This one is even simpler: a piece of 1/8" Masonite hardboard, rough side out, painted the same grey-green, and sprinkled with some ground foam, in this case, the bluish stuff. I added an overspray of grey primer from a can, which was easy to do as the Masonite lifts right out, held in place by the curved "sky" backdrop behind, and the plaster scenery in front.
....
I know that I can model whatever I want and to keep it fun, but one of my desires is to have a layout that, at first glance, looks plausibly realistic.
You're correct in that you can model whatever you want, and, I might add, do a good job of it, too, to which anyone looking at your photos can attest. Just remember, though, that this is a very self-indulgent hobby, and that you need to please yourself first. If you succeed, chances are others will also be pleased by what they see.
I apologise for going on at such length, but your questions caused me to think about the situation, which was a good thing.

Thanks for bringing it up.
Wayne