Dark rings , or discolouration in patches or halo rings , more visible from a distance can be difficult to treat.This fairly rare condition is usually due to bacteria , or rather the action of bacteria on the thatch left on your lawn surface. They leave the soil particles coated in a waxy substance, so the soil gradually becomes unable to absorb water.
It is not confined to drought conditions, even lawns watered correctly through the season, can suffer this problem. If left, it always gets worse, with the grass growth getting poorer and the soil hardening quickly making growth harder.
The solution is to use a wetting agent. I have heard of gardeners having some success with simple surfactants, but it would depend on the chemical acidity of the soil and salts present. A more reliable solution would be one of the available professional wetting agents made for the job, these cut through the wax easily and quickly stop the problem.