I have got a custom meta box for my pages and posts that contributes custom CSS from the textarea. But what I am really after would be to simply have a port feild for one HEX color code which will affect your body tag.

This is actually the code I have got atm

add_action('admin_menu', 'custom_css_hooks');
add_action('save_post', 'save_custom_css');
add_action('wp_head','insert_custom_css');
function custom_css_hooks() {
    add_meta_box('custom_css', 'Custom CSS', 'custom_css_input', 'post', 'normal', 'high');
    add_meta_box('custom_css', 'Custom CSS', 'custom_css_input', 'page', 'normal', 'high');
}
function custom_css_input() {
    global $post;
    echo '<input type="hidden" name="custom_css_noncename" id="custom_css_noncename" value="'.wp_create_nonce('custom-css').'" />';
    echo '<textarea name="custom_css" id="custom_css" rows="5" cols="30" style="width:100%;">'.get_post_meta($post->ID,'_custom_css',true).'</textarea>';
}
function save_custom_css($post_id) {
    if (!wp_verify_nonce($_POST['custom_css_noncename'], 'custom-css')) return $post_id;
    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) return $post_id;
    $custom_css = $_POST['custom_css'];
    update_post_meta($post_id, '_custom_css', $custom_css);
}
function insert_custom_css() {
    if (is_page() || is_single()) {
        if (have_posts()) : while (have_posts()) : the_post();
            echo '<style type="text/css">'.get_post_meta(get_the_ID(), '_custom_css', true).'</style>';
        endwhile; endif;
        rewind_posts();
    }
}

For those who have it employed in the instance you already produced, you are able to duplicate it with a brand new meta box and call the input from that meta box for that tag.

<?php function insert_custom_css2() {
 if (is_page() || is_single()) {
   if (have_posts()) : while (have_posts()) : the_post();?>

        <body style="color:<?php get_post_meta(get_the_ID(), '_custom_css2', true); ?>;">

<?php
      endwhile; endif; rewind_posts();
  }
}?>

Alter the inline type of the tag to what you would like to transmit the Hex to (background, color, etc) then just supply the full hex value when adding a brand new publish / page