Any suggestions how to make a stich look but having it look white on dark fondant?

Paint it on with white food colouring, or a mixture of some kind of white lustre dust and vodka.

I would paint, as mcaulir posted...

OR

I would try a stitching wheel, then paint the little grooves white w/ the tiniest brush...

OR

Pipe stitches...with royal icing....

OR
Extrude or roll an extremely thin snake, cut it up, and glue it in a stitch formation...

I personally would experiment on scrap fondant and do whichever gave me the cleanest, best look, even if it wasn't the easiest way...

AI carefully pipe with RI as mentioned above. I think I'll paint next time if it is a satisfactory alternative.

AThank you :). I will try painting the groves. But What white color product do you use? All my wilton gel colors are everything but white. I never see white gel anywhere

I did white stitching on black fondant when I dd my Chuck Taylor Converse cake.  I talk about the exact process I used on my blog:

http://cupadeecakes.blogspot.com/2012/01/chuck-taylor-converse-shoes-grooms-cake.html

But also, here's a picture of the results:

Hope that helps!!

AAwesome. Thank you for taking the time. In ur blog u said u used PGA What is that exactly?

cupadeescakes...Are you kidding me?! Those are FANTASTIC!!!!