Do you have patches of yellowish-green grass in your lawn? We do in our office landscape as you can see in the image below. Apparently, so do many other people.

The patches of off-colored grass are 'rough bluegrass'. Rough bluegrass got into the fescue seed growing fields in Oregon and ever since commercially available fescue seed has had a small amount of rough bluegrass mixed in.

Since the grass spreads by stolons, the patches get larger over time. Unfortunately, there is little one can do except kill the patches of rough bluegrass and reseed with fescue. If you would like to learn more, visit this NC State TurfFiles article.