This plugin allows to add different colors and background-images to the categories of your favorite WordPress-Blog

You can download this plugin here.

Well, OK. You have to do something for getting the effect.

First: define a new function in your favourite theme, that adds a class to the tag, that includes the category. A samle could be the following:

function the_category_header()
{
    $categories = (array)get_the_category();
    $thelist = '<ul class="categories">';
    foreach($categories as $category){
	$thelist .= '<li class="catColor_' . $category -> category_nicename . '">';
        $thelist .= $category->cat_name;
        $thelist .= '</li>';
    }
    $thelist .= '</ul>';
    return $thelist;
}

Second: Call this function to include the categories into your layout.

Enjoy.