Translate specific WP string.

functions.php

<?php

// Untested, but should give you an idea.

add_filter( 'gettext', 'claire_rename_color_to_colour' );
/**
 * Rename the exact WP string "Color" to "Colour".
 * 
 * Doesn't affect non-exact strings.
 * 
 * @link https://core.trac.wordpress.org/browser/tags/4.6/src/wp-includes/l10n.php#L105
 * 
 * @param string $translation  Translated text.
 * @param string $orignal_text Text to translate.
 * @param string $domain       Text domain. Unique identifier for retrieving translated strings.
 */
function claire_rename_color_to_colour( $translation, $original_text, $domain ) {
  // Target WP strings by excluding anything with a textdomain.
  if ( ! is_null( $domain ) ) { 
    return;
  }
  
  if ( 'Color' === $original_text ) {
    $translation = 'Colour';
  }
  
  return $translation;
}