Did you read this warning in the QPalette documentation?

Warning: Some styles do not use the palette for all drawing, for instance, if they make use of native theme engines. This is the case for both the Windows XP, Windows Vista, and the Mac OS X styles.

So what should we use in place of QPalette for the same purpose.

pratik041 wrote:

So what should we use in place of QPalette for the same purpose.

pratik041 wrote:

I am able to change its color through stylesheet (…)

Seems to me you already answered your own question.