Background colour using Javascript on CRM 2011

Hello,

I'm trying to change the background colour of the backfround that surrounds a field. I'm using the below coding but it only changes the colour inside the field itself, not outside.

//Change background colour of field
function ChangeBackgroundColour()
{
    if (Xrm.Page.getAttribute("new_purchaseprice").getValue() != "")
    {
        document.getElementById("new_retailprice").style.backgroundColor = "yellow";
    }
}

Can someone help please.

Thanks