What Phill means is that when a billboard or painted sign is viewed in real life we can still see the surface it was painted on. A concrete wall painted with a sign still looks like painted concrete, for example.
You can apply a faint texture to the sign itself, then place your logo on top and set it's properties to multiply. This will allow the texture to show through the logo. You can adjust several parameters to get it looking just how you want it. In the end the logo's colours should look a little washed-out, but only because you can see the texture through the logo. If you look at default LFS track signage you will notice various textures backing the signs. Close inspection will show you the textures behind the logos.
"Noise" can be added (in most good graphics programs anyway) with a few mouse clicks. Noise is used in place of an actual texture, and just adds a slightly uneven look to a surface. In real life surfaces are not perfect, especially walls, wooden or metal signs etc, some "noise" can give them a less polished and more realistic appearance.
Although the results aren't as punchy and colourful, it does look more realistic, your signs will stand out but will look more like a natural part of the scene.
