Prasanna,
This type of coloration typically appears as part of a bacterial infection. In the US we use medications like Maracyn 1 and Maracyn 2. I'm not sure if these are available in India, but there may be a similar medication sold under a different name. Maracyn 1 is erythromycin and Maracyn 2 is Minocycline.
Typically these infections aren't spontaneous though, they usually happen after the fish's immune system has been weakened by forms of stress like aggression from other fish or water chemistry issues, so you'll need to address this as well.
What is your water change regimen and have you tested your tank water for Ammonia, Nitrite, and Nitrates?
I do need to be honest with you though, once a fish gets to this point, I haven't had much luck in reversing the course of the illness.