If you are striving for realism on your layout, weathering of structures and rolling stock is probably on your agenda. For the moment, let's not consider the weathering details such as drips, streaks, dents and rust spots.
Instead, just consider the overall dulling and bluing of colors necessary to convey a sense of distance between the observer and the layout. Usually, this is accomplished by over-sprays of graying and bluing colors applied with an airbrush, by washes of ink/alcohol mixtures, by weathering chalks, or by a combination of these.
The trouble is knowing when to stop. It is very easy to overdo the effect and end up with a really drab, colorless scene or model.
Using photos is one way to judge what degree of overall dulling looks best to you. Take a photo of the scene or model to be weathered and make a few different versions of the image using your editing software.
Photobucket and most other photo hosting sites offer color saturation as one of their editing options.
Here are three versions of a scene with decreasing color saturation.
Do this with a number of photos, and you will begin to get a sense of just how much dulling appeals to your tastes. Remember that weathering is very subjective - one man's perfectly weathered model may look drab to another.
By the way, the actual scene on my layout looks to be somewhere between the first two photos in real life. I boosted the saturation slightly in the first photo.
Just an idea for discussion.