wp_get_post_terms requires a post ID, it appears you’re trying to use the term_ID.
You can get your taxonomy values:
<?php if( $portfoliocategories = get_terms( array( 'taxonomy' => 'portfoliocategories' ) ) ) :
foreach( $portfoliocategories as $category ) :
$couleur = get_field('couleur', 'term_' .$category->term_id );?>
<?php echo $category->name.' has couleur: '.$couleur.'<br/>'; ?>
<?php endforeach;
endif;
?>
Code untested. Also worth looking at this page on Loading from a specific term
