We would be wealthy beyond our wildest dreams if we had a good answer for this one. 

Start by looking through magazines or driving around and looking at other houses to get an idea of which direction you want to go.  We have tens of thousands of colors available to look at in our chip racks and can loan you a fan deck to take home.  We can also color match colors from any other paint manufacturer, wallpaper or existing paint into any of our high-quality paints.  Above all, make sure you look at the color you are considering in the room you are painting.  Lighting and time of day can make a big difference so live with it a little while before beginning to paint.