I am going to guess and say you have cool season grass (tall fescue) that is green with some warm season grass (maybe wild Bermuda) mixed in that is now going dormant and brown. See it all of the time. Look at the differences in the blades of grass and Google Bermuda and tall turf type fescue (TTTF) and see if this looks to be the case. If so what will naturally happen over time is the Bermuda will eventually take over the fescue.

Where are you located? I agree with grassboro, but if you are in more northern lattitudes then zoysia would be more likely over bermuda. I live in CT and there are plenty of people that have zoysia patches, or even whole lawns up here that are currently light brown. Either way, you will have to kill the warm season grass if the patches bother you, and if that's the case, then pray that you have zoysia.

"and if that's the case, then pray that you have zoysia"

Now that is funny!! I am still laughing. And the sad thing is, it is true.

Or it could be a mix of Kentucky bluegrass (going dormant in the winter) and fescue, which remains green.