Displaying a different look for a post dependent on Category

I like the category of each post to define how that post looks. A good example of this is on the ThinkProgress blog: http://thinkprogress.org/

I’ve tried a couple things, such as:

<?php if (is_category(‘postrock’)) { ?>
html to display
<? } else { ?>
html to display
<? } ?>

But that doesn’t work. I assume because is_category is for IF we’re on the category page for postrock, then display this.. not IF the category assigned to this post is post-rock, then display this.

I’ve also tried:

<?php if the_category() == postrock ?>

But that only prints the category name.

Any help is MAJORLY appreciated!