OK, Ed, I finally got a chance to catch my breath and look long and carefully at your tires. I say they are perfect as is. As you know, it's really easy, and tempting, to keep going once you get an effect you like. I think for a dry dusty envrionment, they look real, and that's the best complimen I have for any model.
I envisioned darker tire color on the sidewalls and raised portions of the tread, with lighter colors in the recesses, but I have been looking at dirty tires (instead of muddy tires, which we usualy associate with European vehicles) and those seem dead on to me.
My vote, for what it's worth, is leave the tires and move on. AS you said, you can always let it stew for a week and then come back and do something more.
Bill