The Number One Culprit: Drought Stress
If your new turf was laid within the last 14 days and is starting to turn yellow, brown, or gray, 99% of the time it means one thing: It is dying of thirst.
Because new turf has no deep roots, it relies entirely on surface water. If the sun bakes the moisture out of the topsoil, the grass blades will enter 'drought stress' to survive. They shut down chlorophyll production (turning yellow) and the soil base of the roll shrinks, causing visible gaps to appear between the strips of turf.
How to Save It
You must act immediately. Do not rely on a light sprinkle from a watering can. You need to deploy a heavy-duty sprinkler and absolutely soak the lawn. Water it for an hour in the early morning, and an hour in the late evening. If you lift the corner of a yellowing roll, the ground beneath it must be soaking wet and muddy to the touch. If you catch it within the first few days, the turf will rehydrate, root, and return to a vibrant green.