I think what you are talking about is sheen. When paints are wet, they have a really beautiful sheen to them that can flatten out as they dry, depending on how much solvent you are using.

The best way to combat this is to add mediums like linseed oil, stand oil, galkyd, etc to your paint. These aid in thinning the paint and making it easier to work with, but don't dry it out and make it appear chalky and flat the way turpentine/turpenoid/other solvents do.

A classic medium mixture for general painting consists of 1 part oil (stand, linseed, whatever you prefer), 1 part varnish (damar, galkyd, etc), and one part solvent (turpentine, turpenoid, gamsol, etc). To increase drying time, you can add a few drops of Japan drier to this but only a tiny bit or you'll risk cracking and other problems later on! Also, remember that damar varnish does not mix well with turpenoid. Only use it with pure-gum turpentine or gamsol for best results.

As you get used to various mediums, you'll learn what works best for you. I personally love the sheen and quickness of drying provided by varnishes, so my general painting medium tends to be about 70% varnish. This varies depending on what I am painting, however, and if I want to depict, say, chalky gravel, maybe I'll only use gamsol with my paint. If I want to depict something super shiny, maybe I'll only use galkyd.

Fading over time can be a result of light interacting with pigments, and most paints have lightfastness ratings on on the tubes to inform you how they hold up under lights. However, this is not something that would be noticeable in a matter of days or weeks--it can take years for paint pigments to actually fade.

Hope that helped!
Rose

http://effartblog.blogspot.com
http://www.rosebriccetti.com