I have a red scar on my neck as a result of z plasty undergone 6 years back. Earlier the scar was raised and red in colour. But now only redness is left. I have tried lot of treatment but redness still persist

Many people have skin that just doesn't handle scars. I'm one. Even the little bumps from stitches stay for years.
We don't know what 'treatments' you used. I for one wouldn't use any. Maybe aloe cream once in a while, maybe a very occasional cortisone cream. But mostly fresh air, away from tight or rubbing clothing, kept clean and dry.
I do hope you kept it out of the sun. That could be your main problem.
But we also don't know your general health, and that could be another big factor.

In other words, see a doctor for this.