Textured lines in Freestyle

Hi guys,

I am trying some things with Freestyle (parameter mode, BI) and I can’t figure out how to use the linestyle texture option. I made a transparent png with some lines drawn on it, but the line-color shows up in the transparent areas of the png. When I tun down Alpha (3th tab) it not only fades away the color but also my texture.

Is there a way to turn of the color (or make it completely transp.) so that I only end up with the lines in the texture image?