Those patches may be in slightly richer soil--better fed, better watered, or better balanced in some manner. It certainly happened to me for a while. Over time, as I kept adding organic feeding, it evened out--but took some years.

The dark stuff? That is some seriously healthy grass.

The lighter stuff more looks like it isn't quite awake yet. It's been a chilly spring.

I might be inclined to overseed with bluegrass instead of fescue, since bluegrass repairs itself. It's not going to slowly take over very easily--it'll require some overseeding help over time to do that--but it'll decrease the amount of overseeding you have to do over the years as it thickens up.

The only consideration there is that bluegrass does need to be fed on the recommended schedule I give out (Memorial Day, Labor Day [either optionally] October 1, and [or optionally] when growth stops).

With fescue, you can also include the Labor Day feeding as an "either optionally" and only feed once in fall, although it won't look its best if you do that. With bluegrass, twice in fall is really a must (and it's recommended for fescue), and three times really is best to encourage spreading (and I recommend it even for fescue, which benefits from the nutrients).

As to which bluegrass you choose is another story, but you may not want to go down that path at all.

As always @User, thank you for your input. Is the grass in this photo bluegrass or just really healthy fescue? From the little I understand this is bluegrass.

My neighbors on both sides have grass similar to the dark grass in the photo. Kills me because they don't fertilize and I do, but their lawn is lush grass. Then again I inherited my lawn after years of neglect.

I can't tell, to tell you the truth. :-) I'll blow the photo up later on (I'm a bit visually impaired) to inspect the tips of the grass later. If they're like the tips of boats, they're probably bluegrass.