Running javascript for dropdown on page load?

Question

I have a js function which controls the appearance of a select element upon change. However, the one thing i need is for the code to run from start so that when the element loads the selected options class should have already been applied.

css:

  select {height: 50px; width: 80px; border: solid 1px #c8c8c8; color:rgba(0, 0, 0, 0);}
select:focus, #select:focus {
  color:black;
}

.lightgrey {background: lightgrey}
.green {background:green}
.orange {background:orange}
.yellow {background:yellow}
.red {background:red}
.purple {background:purple}

JavaScript:

function colourFunction(select) {
        var colour = select.options[select.selectedIndex].className;

        select.className = colour;
        select.blur();
    }

Select:

<select class="selectElement" runat="server" id="dropdown_" onchange="colourFunction(this)">
                    <option selected="selected" class="lightgrey" value="N">N</option>
                    <option class="green" value="G">G</option>
                    <option class="orange" value="O">O</option>
                    <option class="yellow" value="A">A</option>
                    <option class="red" value="R">R</option>
                    <option class="purple" value="U">U</option>
                </select>

I want the class for N to be applied on load with color:rgba(0, 0, 0, 0);

Answer 1

if you're using jquery:

$(function(){
 //all onload actions you want
});

if you want to stick with pure js

document.onreadystatechange=function(){
//all onload actions you want
}

Answer 2

If you're wanting the JS to run on page load, then you can use the "window.onload" command.

window.onload = function()
{
    colourFunction(select);
};