No, I'm not talking about shimmer dust or lustre dust. Here's a link to the kind of powdered color I'm talking about
:
Food Color- Powdered - Country Kitchen SweetArt
Also, wouldn't you be able to paint with liquid food color also? I haven't tried it, but it could work, as well as using a gel. But I've found that powdered color mixed with an alcohol gives you the most opaque results.
On another note, why can't you pipe royal icing on to your cake for a more realistic looking white stitch? I'd think that would work better anyway.