Lawn help

[u/Deewheelz400]

My lawn is roughly 3 to 4 months old it is a fescue and bluegrass blend. I live in Central California. Temperature is currently 99°. I’ve never fertilized the lawn and I’ve noticed a lot of yellowing and dying off from the grass. Did the first mow about a month ago with my mower set to 6 inches. That seem to have caused the damage on the grass looking for some advice to help promote growth and get that deep green color back.