How to calculate post index when using offset in custom query

In your case, count would be offset + current_post:

$my_query = new WP_Query(
    array(
        'post_type' => 'colors',
        'posts_per_page' => 5,
        'offset' => 2
    )
);
if ( $my_query->have_posts() ) :
    while ( $my_query->have_posts() ) :
        $my_query->the_post();
        echo $my_query->query_vars['offset'] + $my_query->current_post;
        the_title();
    endwhile;
endif;
wp_reset_postdata();