single post category page ?

How can i make a category dependent header outside the loop to display the category name or category id etc. with a single post.
I’ve managed to do this with the category template which displays multiple posts (eg. domain.com/category-a/).
Now i’d like to have the same effect with a single post (eg. domain.com/category-a/post-title-or-something/) page.
The category in my case is a root category, but perhaps it could also be possible with child categories?!