Javascript: "if" form get value is "on"

I'm having trouble making this script function correctly.

I am trying to make the value of a form input show or hide different sections on the action page.

For example, the html code below shows that all header text starts as hidden, but the javascript function should unhide the section based on the GET value of the form.

As a result, to show the "custom" section, the link would be http://ift.tt/2eE4mgy

Here are the sections that need to be shown / hidden based on form values.

<div id="customsection" onload="javascript:customCheck();" style="display:none" class="custom">
    <h1>Custom Text</h1>
</div>

<div id="whitesection" onload="javascript:whiteCheck();"  style="display:none" class="white">
    <h1>White Text</h1>
</div>

<div id="colorsection" onload="javascript:colorCheck();"  style="display:none" class="color">
    <h1>Colored Text</h1>
</div>

Here are my script functions:

function customCheck() {
    if ($_GET['custom'] == 'on') {
        document.getElementById('customsection').style.display = 'block';
    }
    else document.getElementById('customsection').style.display = 'none';

}
function colorCheck() {
    if ($_GET['color'] == 'on') {
        document.getElementById('colorsection').style.display = 'block';
    }
    else document.getElementById('colorsection').style.display = 'none';

}
function whiteCheck() {
    if ($_GET['white'] == 'on') {
        document.getElementById('whitesection').style.display = 'block';
    }
    else document.getElementById('whitesection').style.display = 'none';

}

What is wrong with the script / how can I fix this?

Thanks in advance!