Since the majority of the surrounding grass is suffering badly from being dry, my guess is the part that is gray is dead from not getting any water at all. Put a couple of cat food cans out in the grass and turn on your sprinklers. See how long it takes to accumulate a full can. That is your target time for weekly watering.

Also take a screwdriver and try to stick it in the ground around the yard. Where the grass is really green it should stick in easily. In your dead spot it should not stick in at all. If it does go in easily there, then you have a low spot and the grass died from rotting out (disease). Root rot will cause the top to turn gray from the same lack of water as from not watering at all. When the roots cannot deliver water to the plant, then it looks like it is not getting enough water.

Find the Bermuda Bible online and memorize it. Growing great bermuda is as simple as fertilizing heavily once a month, watering heavily once a week, and mowing at 1.25 inches twice a week.

Thanks for the advice - I had done the screwdriver test a week or two ago - I can get it about 3 to 3.5 inches down everywhere (presumably: sod, thin layer of topsoil, then rock - typical new construction) - including the area w/ the brown spot.

I'm not ruling out lack of water as the culprit, but if it is disease, how would one go about treating it?

This is a 'low spot' in the lawn - which in general suffers from a lot of unevenness (lumpy/bumpy) - so I'm hoping to do a level in the spring.