For the past week or two there has been an active thread in the AVSIM P3D forums about a modification one of the members there developed that increases the brightness of Prepare3d scenery, adds more realistic (and darker) shaders for cloud and scenery shadows, and creates a more realistic horizon haze in the far distance. That thread turned into a new thread in which the user, Pe11e, has put up a little app that gives you some configuration options and creates the ability to restore the original shaders if you like.
Note that this modification is for P3D v3 ONLY. I installed it over the weekend, and it really is a marvelous modification. My flights have never looked so realistic, almost amazingly so. A few folks have reported problems with the installation, so be sure to read the entire thread and to back up your ShaderHLSL folder (which is in the P3D folder structure) before you start.
Best of all, the app is free, although Pe11e does have a donation button on the app which I think is well worth using. These more realistic shaders nearly transform the look of the sim, as you will notice the first time you sit on the ramp and watch cloud shadows so realistically passing over the field (as I captured in the screen shots below).