Using this trick, when the author of post reply in comments, his / her comment box will have different color or background. We have to manipulate the main style sheet for this. Find out where the css style is defining comment box style. As it may vary from theme to theme, we are a prototype set, so that you can understand the matter and do it for your WordPress blog.
The declaration or value marked with red color, needs to be changed by you. Just like the tricks we mentioned while customizing blockquote, you can add background image and several other tricks too.