What medium are you talking about? Gel medium? I have a few containers of various mediums around, and I think one of them is gel. What are the advantages of using medium to thin paint, rather than simply using water? I've heard about this transparent shadows, opaque lights approach before. It seems like a method that a lot of artists like to use. Also, after looking at a few tutorials in the fine art section, it seems that a method many painters like to use is to just put tiny dabs of color on the canvas just as "place holders," so that they can see how their color relationships are building up without having to cover everything with paint right from the start.
It's usually good to use a lot of medium for the shadows and let the warm underpainting show through them a little to give them a nice rich depth.