I agree with GrassDaddy that this is a good product, but I did have some issue with whitening of existing grass in a very recent overseed two weeks ago. Not everywhere, just some small areas here and there. It may have been poa that I didn't recognize, but I also have some fine fescue that, as GrassDaddy said, is susceptible to some whitening as well.
Good news, though. It's been two weeks, and I just mowed today. There's almost no trace of whitened grass anywhere in my lawn anymore including in any areas that are more heavy fine fescue. The lawn looks absolutely great, and I've seen a noticeable decrease of the few weeds lingering in my lawn.
I applied the Scott's at 3/4 bag rate along with Milo the very same day at full bag rate (I usually go more than the bag rate for Milo, but since there was plenty of N form the Scott's Starter Fert, I went lighter than usual).