$string = 'apple:paper:red,pear:rock:blue,peach:scissors:green';
$result = array();
foreach (explode(',', $string) as $piece) {
    $result[] = explode(':', $piece);
}

$string = 'apple:paper:red,pear:rock:blue,peach:scissors:green';
foreach (explode(',', $string) as $key=>$piece) {
    $result[$key] = explode(':', $piece);
}