Как это?
$a1[0] = ['Color:red','Color:green','Color:blue'];
$a1[1] = ['Size:32','Size:34','Size:35'];
$a1[2] = [5,4,8];
$string = [];
for ($i = 0; $i<count($a1[0]);$i++){
$string[$i] = $a1[0][$i].','.$a1[1][$i].','.$a1[2][$i];
}
return print_r($string);