The following line is missing a semicolon at the end (and instead has it inside the SQL statement), so the PHP is not executing correctly.
$query="SELECT Color FROM UserColors JOIN Users ON UserColors.idUserColors=Users.UserColors_idUserColors WHERE user_name='$login_session';"
It should be:
$query="SELECT Color FROM UserColors JOIN Users ON UserColors.idUserColors=Users.UserColors_idUserColors WHERE user_name='$login_session'";
Also, you should be using mysqli or PDO. Using mysql_query in PHP is deprecated, as seen on the mysql_query PHP page.