When you click on an item in a listbox it refuses to change the colour of the label even if it's set. Looks like a bug to me - it works however with a textbox.

code:
$listbox = $form.ListBox("")
$listbox.Size=110,255
$listbox.Location=15,80
$listbox.BorderStyle=1

$listbox.AddItem("My Item")
$listbox.OnClick = "Display()"

function Display()
$text = $form.Label("This my item",140,80,200,16)
$text.BackColor = 255,255,255
endfunction