Web Forms :: Coloring DropDown ListBox Items?

I have a situation where some dropdown listboxes contain some items, then there's a separator line, and then there are more items. The ones above the separator line reflect the "commonly used items". To better highlight these I was using code like this to individually color those specfic items:

foreach (ListItem item in ddList.Items)
{
if (item.Value == "-1")
unAssigned = true;
if (!unAssigned)
item.Attributes.Add("style", "color:#5558ff");
}

This code works fine EXCEPT that once a PostBack occurs then the highlighting disappears. The items in the dropdown listbox remain (without repopulation) but not the highlighting.

I'm curious why this is happening and whether there's a way for the highlighting to "stick" short of calling this code from the Page_Load event handler every Postback?