Styling different categories

Hi,

I want to have a different background-color on my post depending on what category it is.
What I need is a variable or funtcion that returns either the catergory ID or the category name of each post. For example the “the_ID()” function returns the post-ID.

I want to make something like this:
<?php
if( the_category_ID() == 2 ) {
echo “<div class=’blue’>”
}
?>

…or something like that ??
This doesn’t work because there is no the_category_ID()-function. Is there a function (or maybe a variable) that i can use?