Why does grass turn Brown?

Question

A detailed explanation with chlorophyll, etc. please.....

Answer 1

After the seeds fully mature the stems and leaves of the grass dies and the nutrient rich chlorophyll is broken down by the plant and relocated into the roots (this process is called resorption).  The roots of most grasses are perennial, so they will use that energy for new growth next year.  The remaining grass is brown because you are seeing the lignin--the tough structural parts of grass.

A similar process happens when the whole plant dies, except the chlorophyll will be scavenged by other organisms (bacteria, fungi, etc.).