Multiple occurrences in $array1 are all treated the same way. This will output :
Array ( [1] => blue )
Notes
Note: Two elements are considered equal if and only if (string) $elem1 === (string) $elem2. In words: when the string representation is the same.
Note: This function only checks one dimension of a n-dimensional array. Of course you can check deeper dimensions by using array_diff($array1[0], $array2[0]);.