Q:

nextSibling instead of nextElementSibling?

This isn't exactly a problem in the code, but I cannot understand the reason behind my problem. First, please have a look at the code:
HTML:
<!DOCTYPE html>
<html lang="en-US">
    <head>
        <title>Head First : Javascript</title>
        <meta charset="UTF-8">
        <link rel="stylesheet" type="text/css" href="css.css">
        <script type="text/javascript" src="cookie.js"></script>
    </head>
    <body>
        <div id="row1">
            <p>False</p>
            <p>False</p>
            <p>True</p>
            <p>True</p>
            <p>False</p>
        </div>
        <div id="row2">
            <p>False</p>
            <p>False</p>
            <p>True</p>
            <p>False</p>
            <p>False</p>
        </div>
        <div id="row3">
            <p>False</p>
            <p>True</p>
            <p>True</p>
            <p>True</p>
            <p>True</p>
        </div>
        <div id="row4">
            <p>True</p>
            <p>False</p>
            <p>False</p>
            <p>True</p>
            <p>False</p>
        </div>
        <script type="text/javascript" src="javascript.js"></script>
    </body>
</html>

CSS:
p{
    display:inline-block;
    width:50px;
    margin:30px;
    text-align:center;
}

JS:
function setElementColor(){
    var numberRows = parseInt(window.prompt("Enter total number of rows that you are seeing on the screen."));

    for(var i = 1; i <= numberRows; i++){
        var firstElement = document.getElementById('row' + i).firstElementChild;
        var counterElement;
        //For now, total elements in 1 row = 5
        for(var q = 1; q <= 5; q++){
            if(q == 1){
                firstElement.style.backgroundColor = "red";
                counterElement = firstElement.nextSibling;
            }
            else{
                counterElement.style.backgroundColor = "green";
            }
            counterElement = counterElement.nextElementSibling;
        }
    }
}
setElementColor();

Now, this might be a stupid question, but it will make sense (I believe). The code that you are seeing is correct, and works without a doubt. But, can you see that nextSibling property in the first if statement? That is what is confusing me. 
Now, all paragraphs have spaces between each other, so this means that all browsers will see those spaces as textNode (except IE). Now, by the means of logic, there should have been a nextElementSibling property there, so that the code looks forward for the next Element, ignoring comments/textNodes, but I tried it, and that is not working. 
Can someone give me a satisfying reasoning behind that?

A:

In your first if, counterElement is set to the nextSibling, being the textnode
if
{
   counterElement = firstElement.nextSibling; //now counterElement is the textnode
}
else
{
}
counterElement = counterElement.nextElementSibling; //no matter if the counterElement is a textnode or a <p> the next element in the sibling tree is a <p>

But regardless of q and the if..else branch taken, immediately after the if..else block, counterElement is set to the next Element sibling, so that regardless if the starting point is the textnode or the containing paragraph, the next element will be the sibling paragraph. The code would behave the same if you would set counterElement = firstElement in the q == 1 block

As an aside, to get rid of the fixed upperbound (columns) and loose the if..else at the same time, you could also use something like
for(var i = 1; i <= numberRows; i++){
    var el = document.getElementById('row' + i).firstElementChild;
    var q = 1;
    while (el!=null){
        el.style.backgroundColor = q++ == 1 ? "red" : "green";
        el = el.nextElementSibling;
    }
}

