Where do you live?
What kind of grass do you have?
What is your watering regimen (frequency and duration)?

50 mm is too short unless you have bermuda, creeping bentgrass, or centipede (and you don't have any of those). Raise the mower to 100mm and the grass will be happier.

If this is Kentucky bluegrass, it has a tendency to go dormant in the summer heat. Since it is brand new turf, it will be a little more sensitive.

You can fertilize with organic fertilizer at any time to improve the color and health of the grass. Don't use chemical fertilizers in the summer.

It appears some of the KBG individual plants were damaged or did not root and have died. It happens with new sod, Do use an organic fert this summer and make sure the turf is sufficiently watered (about an inch per week) and raise your mowing height this summer. Most importantly for KBG--winterize this Fall and return to 2-2 1/2" (75mm?) onec high temps drop to 75 degrees F. Next Spring all should be well.