I can't definitely answer that one with 100% reliability, but from my personal experience as a creator of Windowblind skins, the darker skins get more downloads.  I don't let that hinder me, though; if you visit my gallery, you'll see I'm a big fan of color.  You can also use Windowblinds, itself, to adjust the color, brightness and saturation of a skin in the Colours tab (video below).  You can also add a texture that is a solid color and apply that, to completely color a skin in that color. (Video on how to do that here: http://www.youtube.com/watch?v=AusBI6rXzmw )