set bgcolor in window.open or document.write or...

[Nat]

Hi I have following code:
-----------------
protected override void Render(HtmlTextWriter w)
{
if (helpString!=null)
{
w.Write("<span class='tooltipiconouter'><img title =
'"+this.helpString+"' class='tooltipIconInner' align='center' src =
'../Lib/images/questionmark.gif' a href='#' onclick = 'myWin =
window.open(\"\", \"\", \"width=300, height=200, directories=no\");
myWin.document.write(\"" +helpString+"\");'></a></span>");
}
}
-----------------

Now I want to change the bgcolor in the window that opens, what and
where do I have to write that?

I've tried myWin.document.bgcolor=#333333 but either it doesn't work,
or it is because i'm writting it in a wrong place.

Please help

Nat