how to add an editor to custom fields in the admin section?

hi there. I want to set up a custom field in the admin section of WordPress so that a client can go in and edit the content of the custom field without having to hard-code things with HTML.

Does anyone know a quick/easy way to do this?

For further clarification, I bring in custom fields to my page like this:

<div id=”greybackground”> <div id=”greycontainer”>
<?php echo get_post_meta($post->ID, ‘greycallout’,true) ?></div></div>