craftgrrl | Washable pens for marking seam allowances on dark cotton?

I'm making a somewhat complicated -- well, complicated for me -- quilt for my friend's son. I need to mark the seam allowances on the wrong side of the fabric so I know where to sew. Usually I use a piece of thick tape on the bottom part of my sewing machine; I put it as far away from the needle as I need so that when I'm sewing a straight line on the edge of fabric it acts as a sort of a guide. This time, however, the seam needs to be about 1/4" away from the center of the fabric that I'm sewing (and later cutting apart). It's hard to explain, I guess, but my *real* question is this: what do you use for marking on dark cotton fabrics? I've been using quilting pencils that wash out, but they just aren't dark enough on this fabric unless i go over it a million times, which gets really frustrating.

And something I made last week from a cardboard box, scraps of batting, and leftover fleece from pajamas:

Kitty bed!!

(My cat doesn't usually look that weird -- this picture was taken *right* after I finished the bed, and he was kind of mad that I made him sit in it. Now he loves it!)