I agree entirely. Mono-color minifigs tend to look more bland, as Bo said, because the monotony of color and tone make the details stand out less. I try to pick a palette of colors to work with, such as Tan, Dark Tan, and Reddish Brown. Or Dark Tan, Green, and Dark Green. Anything, really, that will allow details to be contrasted and so more visible.
And as far as the pieces not being available in certain colors, you can always sponsor a color.http://www.brickforge.com/?page_id=657
It's not cheap by any means, but if you have the capital and the need, it's always there as an option.