marking a post that was sticky on category template

Question

Check if »sticky«

There’s the is_sticky()–Conditional Tag, which you can read about in core here.

How-to mark as »sticky«

This works for posts, that have the “stick to the front page” check box checked:

How »sticky« works internally

So basically the function checks if the post-ID is inside get_option( 'sticky_posts' );. That means that you can’t question if a post was “sticky”. You can only check which posts are currently “sticky”.

Style as many »sticky« posts as you need & style »sticky« them specially

So your only chance would be to show only the number of sticky posts that you need ¹⁾ on the front page and skip the rest. Then you can apply to all your archive posts the post_class(); (inside the loop). This adds different classes to the post, but also the .sticky class for “sticky” posts. Which means, that you now have a way to style them differently.

.sticky {
    color: #FF00FF;
    background-color: #009ee0;
}

^{1) You can get the newest sticky posts with $newest_stickies = array_slice( rsort( get_option( 'sticky_posts' ) ), 0, 2 ); – change the last number to the number of posts you need. To add them to the loop, use 'post__in' => $newest_stickies as argument for your main/front-page query.}