Your shadow base does not neccessarily have to have any color - it could be transparent or nude (skin tone), if that's the case, you can pretty much use it under any shadow colors you are applying (and it won't change the original colors of the shadows, but prevent it from creasing and help intensify the color).
You can also use a variety of colored bases (i.e. paints, paint pots, CCB) to bring out the colors of a look (i.e. use a green colored base to bring out a gold/green look, etc)