Sir,
I am a horticultural supervisor in a reputed firm and my lawn become brown due to extreem cold. How can I make them green in a short time?

Do you know what type of turf you have? Some grasses go dorment once you get frost.

Is not this your field of study? But in general grass turns brown after a frost and is not really green all winter unless you have special turf put in that is designed to do well in the winter.

So I would hire a expert in the field to come in and re-turf your lawn

The type of turf will make it brown in the winter, so you will need to know what grasses make up your lawn, and have the correct type of lawn for your area.

Fr_Chuck, I had that same thought but lacked the tactful way of stating my view.

Your profile doesn't give your location. Yes if you are new to freezing weather, some varieties of grass do turn brown at the first frost. It is more prevalent in the heat resistant ones. I guess if you really want green grass before spring, you could find the green paint applied to Christmas trees that yellow after the frost.

Your sod isn't dead, just dormant, and as a Horticulturist you should know this. Similar to the leaves on your Maples and Oaks falling off the tree during the winter months. No shame in that. Paint, new sod or a quick seeding of annual rye if you must have green growth for some reason.