Insert 5 different classes to first 5 posts (then rotate classes)

Yes!

Have a look at this post and the section content color, the post has a download so you can see the code!

It uses color-1, color-2, color-3, color-4 and color-5, color-6 and color-7, adding the class to the post class:

So for 5 you would just change the $divisor to 5 like this:

<?php
/* Set the class based on the modulus of the current_post counter
   $counter will be 0 to 4, we add 1 to current_post which is zero based */
global $wp_query;
$divisor = 5;
$counter = ($wp_query->current_post + 1) % $divisor;
//Add the classes to the $class variable
$class = $counter ? 'color-' .$counter : 'color-' .$divisor;
?>

<?php /* Comment: We have added the $class to the post_class() */ ?>
<article id="post-<?php the_ID(); ?>" <?php post_class( $class ); ?>>

HTH

David

<?php
$extra_classes = array('red','orange','yellow','green','blue');
$extra_class = $extra_classes[$wp_query->current_post%count($extra_classes)];
echo $extra_class;
?>

within the loop of index.php, you can either echo the css class in a post div:

<div class="<?php echo $extra_class; ?>"> ...... </div>

or use it in (the possibly existing) post_class():

<div <?php post_class($extra_class); ?>> ...... </div>

edit:
thanks, @david, for explaining the modulus operator ??

Sorry this is super late reply. At that time, I opted to use css to have a workaround, using the nth-child selector which is a bit problematic if I want to consider browsers that don’t support it.

Now I’m coding a theme with a similar function, so I’m gonna do it right now with your suggestions! Thank you!

Edit: btw, alchymyth’s suggestion worked for me.