Right now, on the G35, for wty purposes, I change it when I feel like it between 5k and 7.5k miles. Once the wty is up, I'll adjust to an interval suggested by UOA, which I predict will fall out at around 10k. In the wife's Sequoia, a short tripper, I'm probably going to settle out at one year intervals, though I might make it nine months (safety margin) based upon what the next UOA says. If I had a turbo like pscholte, I'd go shorter too.
Don't fret over color do at least one UOA for yourself so that in the future, you will believe that the darkening is not harmful. You're not talking quickly smut black like a diesel, are you? That might be cause for some concern, and not about the oil.