<?php
$query = "SELECT TIME_TO_SEC(TIMEDIFF(NOW(), FROM_UNIXTIME('jointime')))/(3600*24) AS 'join_time' FROM users WHERE userid =" . $uid;
$results = $db->query_first_slave($query);
if ($time_inactive < $year){
  if ($results['join_time'] > 0 && $results['join_time']<1) { 
        $color['fajoin_time'] = 'orangered'; 
  } elseif ($results['join_time']<2 && $results['join_time']>=1) {
        $color['fajoin_time'] = 'darkorange';
  } elseif ($results['join_time']<3 && $results['join_time']>=2) {
        $color['fajoin_time'] = 'lightgray';
  } elseif ($results['join_time']<4 && $results['join_time']>=3) {
        $color['fajoin_time'] = 'limegreen';
  }
    elseif ($results['join_time']<5 && $results['join_time']>=4) {
        $color['fajoin_time'] = 'blue';
  }
    elseif ($results['join_time']<10 && $results['join_time']>=5) {
        $color['fajoin_time'] = 'indigo';
  }
    elseif ($results['join_time']>=10) {
        $color['fajoin_time'] = 'black';
  }
  else  { 
    $color['fajoin_time'] = 'red'; 
  }
}
else  { 
    $color['fajoin_time'] = 'red'; 
}


$badgejs .= 'badge["fajoin_time"] = "' . $color['fajoin_time'] . '";';  
$badgejs .= 'badge["fajoin_timeval"] = "' . number_format($results['join_time']) . '";';

jQuery: I haven't changed "chess-rook" name here, you could please select it as per availability of icons.

$('.fa-chess-rook').css("color",badge["fajoin_time"]);
$('.fa-chess-rook').css("cursor","pointer").attr("title", badge["fajoin_time"] + " Days in UNIX.com forums");
$('.fa-chess-rook').closest('div').find('.fa-circle').css("color",badge["fajoin_time]);