A couple of ways you can do it:
1) Try some of the "Clear" Tamiya paints. They come tinted blue, red, green, yellow, or orange (I think that's all) but are clear.
2) Use food coloring mixed in Future until you get the depth of colro that you like.
Spray either one in light colors until you get the shade and depth that you like. I'd practice on a piece of scrap before going to the real thing though.