Custom query filtered with GROUP BY not working

I’m trying to get the posts from a Custom Post Type and list the contained custom fields as types so that if I have for example four posts with the custom field “fruit” set to “banana” I get a list showing one “banana” and not “banana” listed four times. Its to build a legend, a key, for the content.

From what I’ve been reading the best way to do this is using GROUP BY in a filter around my custom query but I can’t seem to get it to do anything and don’t know how to bug test query calls to the database.

I tried StackOverflow but no response.

My code is…

function sort_legend ( $groupby ) {
    global $wpdb;
    $groupby .= "{$wpdb->postmeta}.meta_key = 'Color' ";
    return $groupby;
}

…in my functions.php and…

<?php  add_filter('posts_groupby', 'sort_legend'); ?>
<?php if ( $myQuery->have_posts() ) while ($myQuery->have_posts() ) : $myQuery->the_post(); ?>
<?php remove_filter('posts_groupby', 'sort_legend'); ?>

…around my custom query.

Any suggestions, or does anyone think there’s a simpler way to do it?