
The HTML document used to display Popup windows is, by default, assigned a black background for both the HTML document itself as well as the SWF. Because of this, background colors other than black will reveal (for a split second) a black window background before displaying your assigned color.
To alter this behavior so that a consistent color is always displayed, open the pop.html document in any ASCII/Text editor (like Notepad, Dreamweaver, TextWrangler), and look for the following in the document header:
<style type="text/css">
body {
margin:0;padding:0;
background:#000;
}
</style>
Modify the background hex value with the color you're using for Background Color (Appearance: Popup).
Then scroll down a little and look for this (also in pop.html):
var params = {
quality: "best",
bgcolor: "#000000"
}
Modify bgcolor with the same hex color value used above.
Save pop.html and reload.