IME the patina gets dark FAST where the mustard is thinnest. You can create patterns by inverting the design, so to speak. Put a thick layer on, and then work quickly to rub down where you want it darker, and wait a few minutes. I did this with a parer recently to put some angled stripes on it, and it worked great.
If you really wanted to make something exact, you could always make a stencil out of good masking tape. If it's pressed down really well, and you have a really fine scratch pattern in your blade, I don't see the mustard bleeding through. You could warm the blade up, do mustard for the grey/brown colour, let it sit. Clean it off and set it with mineral oil for a day or so. Follow that up with a nice red meat patina, set in between your mustard pattern and you'd get a great blue and grayish, interwoven look to the blade.