Incrementing a class in a custom walker

Question

I have written a custom walker that I am using with wp_nav_menu but I need to iterate through 4 custom classes, and I'm not sure how to do this. Basically, each menu-item <li> needs to be given a class of color1, color2, color3 or color4, cycling back to color1, lather, rinse, repeat, in that order.

Ordinarily I would do this with a $i and a $i++, but how do I do this in the Walker class?

EDIT: My Walker class code:

class Salamander_Advent_Walker extends Walker_page {
function start_el(&$output, $item, $depth, $args) {
    if ( $depth ) {
        $indent = str_repeat("\t", $depth);
    } else {
        $indent = '';
    }

    $advent_thumbnail = get_post_meta($item->ID, 'advent-thumb', true);
    $advent_slug = get_post_meta($item->ID, 'advent-slug', true);
    $advent_oneliner = get_post_meta($item->ID, 'advent-oneliner', true);
    //$description = get_post_meta($item->ID, 'advent-thumb', true);

    $output .= $indent . '
    <li class="active color4">
        <a href="#day'. $advent_slug .'">
            <span class="day">
                <strong>'. $advent_slug .'</strong>
                <span>&nbsp;</span>
            </span>
            <span class="content">
                <small>'. $advent_slug .'</small>
                <img src='. $advent_thumbnail .' width="126" height="91" alt="advent" />
                <strong>'. $advent_oneliner .'</strong>
            </span>
        </a>';                      
}

}