var arr = [2,5,8,11,14,17,....];

<ul>
     <li>testing</li>
     <li>testing</li> // -->2nd li be green
     <li>testing</li>
     <li>testing</li>
     <li>testing</li> // -->5th li be green
     <li>testing</li>
     <li>testing</li>
     <li>testing</li> // -->8th li green like this
     <li>testing</li>
     <li>testing</li>
     <li>testing</li>
     <li>testing</li>
     <li>testing</li>
    <li>testing</li>
</ul>

arr.forEach(function(num){
   $('ul li').eq(num + 1).css('backgroundColor', 'green');
});

li:nth-child(2) {
    background: #ff0000;
}


li:nth-child(3n+0) {
    background: #ff0000;
}