You, sir, are a heretic.

Seriously, I tie 95% of my flies with eight ought grey thread. The thin diameter, and the neutral color don't seem to matter to the trout. The only time I use another color is if the thread is part of pattern (i.e. Sierra Bright Dot) or if the head or collar is important (i.e. Elk Hair Caddis or Amy's Ant). It doesn't matter if the fly is black or white, I use grey thread. The materials cover it up most of the time anyways.