Is there any particular reason for having the ground represented by brown on an artificial horizon or PFD?
Blue represents sky, brown represents earth, which will be used for easily identifying the aircraft attitude (along with line of origin). Using blue to represent the sky makes conventional sense. But why is brown used? Is there any scientific reason for using brown instead of another color?
The answer to the specific question of "why PFD's (glass cockpits) use this standard" is because their old style gauge cockpits used it before them. The glass cockpits of today are made to be similar to the older gauges so as to avoid confusion in their operation when moving from one type of aircraft to the other. This has been shown to be an issue in the past in the difference between American and Russian attitude indicators as such instrument operation strives to be at least somewhat uniform.
Now as for the more general question "why are attitude indicators Blue/Brown?" there are a few reasons. First off some types of color blindness cause issues with the blue/green mix making it a combination to avoid (some info on color blind flying). Second, and perhaps as important is that blue and brown sit basically on opposite sides of the color wheel making them very contrasty while blue and green sit right next to each other on the wheel. This ensures accurate viewing in most lighting situations.
There are also other attitude indicators like those commonly used in military aircraft which have had different colors
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The oldest examples are completely void of any color (you can see why they introduced color)
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For what its worth some modern flight displays now include terrain data and are capable of showing many other colors to outline it as can be seen here on this Garmin unit
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If they used green (like grass) you would have asked the same: "Why didn't the use brown (like soil)?"
Blue - brown is easier to distinguish, than blue - green, blue - red or another color-combination you can think of.
Contrast:
The use of this two colors can be explained by the high contrast from blue to brown. The pilot can find out with one quick glance at the PFD how the aircraft is positioned in the air.
Natural colors - pilot instantly knows what it means:
Because those are "natural" colors, the pilot instantly knows that blue means sky and brown means ground. He doesn't have to process the color information in his head just to find out what it means. This would take much longer.
