Q:

Please check a newbie's script

Please check my script below. 
Im trying to add some color to some of the text... this is what i have managed to work out thus far.. is it correct? 
<center> You will be redirected to the number one proven training system in <span style="color:blue;font-weight:bold"> <span id="countdowntimer">12 </span> Seconds </span> </center>

<script type="text/javascript">
var timeleft = 12;
var downloadTimer = setInterval(function(){
timeleft--;
document.getElementById("countdowntimer").textContent = timeleft;
if(timeleft <= 0)
    clearInterval(downloadTimer);
},1000);

A:

Yes, the text of the countdown is blue if that is what you mean.  Ifyou want to change the color when the timer runs out try something like...
if(timeleft <= 0)
clearInterval(downloadTimer);
document.getElementById("countdowntimer").style = "color:red";

You might find tools like https://jsfiddle.net/vnjgngo1/ helpful in the future.  In this fiddle you can paste your code and html in two separate blocks.