$colors = get_field( 'cor' );
// Create a comma-separated list from selected values.
if ($colors) {
$boxes = array();
foreach ($colors as $color) {
$boxes[] = '<span style="height: 20px; width: 20px; margin-right: 3px; position: relative; background-color: '.$color.';"></span>';
}
echo implode(', ', $boxes);
}