Changing background color of text box input not working when empty

You didn't call the function and you have other errors, should be:

function checkFilled() {
    var inputVal = document.getElementById("subEmail");
    if (inputVal.value == "") {
        inputVal.style.backgroundColor = "yellow";
    }
}
checkFilled();

Fiddle

You were setting inputVal to the string value of the input, but then you tried to set style.backgroundColor on it, which won't work because it's a string, not the element. I changed your variable to store the element object instead of its value.

Try this:

function checkFilled() {
var inputVal = document.getElementById("subEmail");
if (inputVal == "") {
    inputVal.style.backgroundColor = "yellow";
    }
}

DEMO --> http://jsfiddle.net/2Xgfr/829/

HTML

<input type="text" id="subEmail" onchange="checkFilled();">

JavaScript

 function checkFilled() {
    var inputVal = document.getElementById("subEmail");
    if (inputVal.value == "") {
        inputVal.style.backgroundColor = "yellow";
    }
    else{
        inputVal.style.backgroundColor = "";
    }
}

checkFilled();

Note: You were checking value and setting color to value which is not allowed, that's why it was giving you errors. try like the above.

on body tag's onLoad try setting it like

document.getElementById("subEmail").style.backgroundColor = "yellow";

and after that on change of that input field check if some value is there, or paint it yellow like

function checkFilled() {
var inputVal = document.getElementById("subEmail");
if (inputVal.value == "") {
    inputVal.style.backgroundColor = "yellow";
            }
    }