Could you use something like the below:
$args = array(
'numberposts' => -1,
'post_type' => array('post', 'page'),
'meta_key' => 'colours', // your existing colur select field
);
// query
$the_query = new WP_Query( $args );
if( $the_query->have_posts() ):
$colours = array();
while( $the_query->have_posts() ) : $the_query->the_post();
$colours[] = get_field('colours');
endwhile;
endif;
// remove duplicates
$filtered_colours = array_unique($colours);
if ( $filtered_colours ) :
foreach ( $filtered_colours as $colour ) :
echo $colour;
endforeach;
endif;
Depends on how you need to show/use the values I guess
