Some of the grass grows in brown along with green grass. I typically see it with fescue. It is just scattered blades, not sections or spots. I cut between 3 and 3.5", I was thinking that maybe the height and some of the grass was laying over and getting a bit sun burnt, or it needs to be aerated because it is choking a bit? They all have the lawn fertilized and have sprinklers and I sharpen my blades every day.