Even light colored designs must be printed on white paper. The inks used in inkjet printers assume you are putting the ink down on white paper. A light color printed on clear paper will be barely visible.
The problem making decals on white paper is cutting them out accurately. Since the paper is white all over, if you cut even slightly outside of the line, you get a white fringe.
One work-around is to use an art program that allows you to set a background color (most decent graphics programs and photo editors do). You then set the background color to match the color the decal will be applied to. That way, you can cut somewhat outside the lines and it will not be real noticeable. You can, for instance, put white letters on the background and cut around the whole word or set of letters/numbers, not cut out each individual letter. I have made white or light-colored decals many times using this technique and it works well.
I am building a couple of models now that I intend to document, which will have white decals. I hope FSM will buy the article I write on this process :-)