List of categories instead of dropdown menu selector in admin options

In that case you can get the list of categories and for each category create a text field something like this:

$categories = get_categories('hide_empty=0&orderby=name');
foreach ($categories as $cat){
    $options[] = array( 
    "name" => $cat->name,  
    "desc" => "Select a color for ".$cat->name." category",  
    "id" => $shortname."_cat_".$cat->term_id,  
    "type" => "text",  
    "std" => "");  
}