<?php  $mysqli = new mysqli('localhost','root','xxxx','xxxx');
    $myArray = array();
     if ($result = $mysqli->query("SELECT CONCAT(Datum , ' ', Tijd) as Datum, KleurL FROM metingen order by Datum ASC limit 20")) {
            $tempArray = array();
            while($row = $result->fetch_object()) {
                            $tempArray = $row;
                            array_push($myArray, $tempArray);
                    }
            echo json_encode($myArray,JSON_NUMERIC_CHECK);

            $fp = fopen('resultsv2.json', 'w');
            fwrite($fp, json_encode($myArray,JSON_NUMERIC_CHECK));
            fclose($fp);
    }

    $result->close();
    $mysqli->close();  ?>

[{"Datum":"17-01-2019 10:31:39","KleurL":17.68},{"Datum":"17-01-2019 11:10:59","KleurL":71.76},{"Datum":"18-01-2019 08:40:41","KleurL":70.7},{"Datum":"18-01-2019 10:30:01","KleurL":71.03},{"Datum":"18-01-2019 12:05:46","KleurL":70.56},{"Datum":"18-01-2019 14:31:58","KleurL":16.2}]

[
[
    17.68,
    17-01-2019 10:31:39
],
[
    18.11,
    17-01-2019 11:15:20
]