WP, PHP: Add classes for every (nth) element

do_color_classes.php

/************* ADDING CLASSES TO POST_CLASS **************/
// every post has a few uniquely colored elements, rather than rely on CSS nth-magic, and to keep things smooth
// and not conflict w/ trying to use jQuery nth-magic in tandem w/ the endless scroll,
// the option is to get the server to do the heavy lifting,
// ideally it would be nice to pass the array of classes to iterate thru into the filter,

/**
 *
 * @param $classes
 * @return array
 */
function do_color_classes( $classes ) {
    // globalizations
    global $wp_query;
    global $array_index;

    // define an array of class names to iterate over
    $classes_array= array( 'yellow', 'red', 'blue' );

    // get the post count
    $count = $wp_query->current_post;

    // set the array index value
    if ( $count == 0 )
        $array_index = 0;

    // compensate for it starting @ 0;
    $count = $count++;

    // pass the class in to match the index.
    $classes[] = $classes_array[$array_index];

    // check the modulus, divide the post count by the length of the array,
    // this'll reset the index counter, and keep iterating thru the $color_classes
    if ( $count % sizeof($classes_array) == 0 )
        // reset the index
        $array_index = 0;
    else
        // +1 to the index
        $array_index++;

    return $classes;
}
add_filter( 'post_class', 'do_color_classes' );