I've wrestled with thread size and color myself, and finally come to realize that all the saddle-makers and other leather workers I know are right. White linen is best. It seems counter-intuitive, but white stitching will not be nearly as prominent as the darker thread. Size is important...I can't trust anything less than 5-ply linen, and that is fairly thick. I prefer 6-ply, and I still break that on occasion.

For those not familiar with this kind of stitching, you have to remember that there are no knots. The tightness of the threads within the awl holes is all that holds it all together. Pounding the stitching after you're done helps tighten everything up, but you still need some heft in the threads or it just won't hold up to constant use and stress.

The only time I use anything other than white is when I'm stitching black leather, or leather that I am going to dye black. Then I'll use black thread. Other than that, I stick to white.

Just my $.02.