I can not update a specific column in DB

I am using input type color as i want to save selected color to DB.
I can see color which I did choose in console (check js), but something  gets wrong with ajax as I can not get echo "nnnnnnnn", but just "mmmmmmmmmmm" (check php).
So I might not debug forward in case of other errors.


php

<?php
echo "mmmmmmmmm"; 
if(isset($_POST["uColorIsSet"])){
 echo "nnnnnnnnnnnn";    
include_once("/wp-content/themes/n4/php_includes/db_conx.php");
$uColor2 = mysqli_real_escape_string($db_conx,$_POST['uColor2']);
$sql = "UPDATE users SET yourColor='$uColor2' WHERE username='$u'";
$query = mysqli_query($db_conx, $sql);
}
?>

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and js

function updateAll2(event) {
var uColorIsSet=0;
 var uColor2 = event.target.value;
 console.log("selected color"+uColor2);
$.ajax({
  method:"POST",
  url: '/wp-content/themes/n4/nF4/user.php',
  data:  {
    "uColorIsSet":1,
    "uColor2":uColor2
    },
    datatype: "text",
    success: function(strdate){
   
      console.log("aaaaaaaaaaaaaa");
     
     },
     error: function(error, txtStatus) {
      console.log(txtStatus+" error on load of canvas on user page");
      console.log('error');
    }
 });
}

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Can you see from data that I  provided what is the problem?


Just for  Info: I am doing the initial setting of color as next.
      

$sql = "INSERT INTO users (yourColor)    VALUES ('$c')";

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