I think you are looking for the panel.alpha value
here is from the help file about it.
Example:
my_panel.transparent = ON;
my_panel.alpha = 0; // Panel 100% transparent
while (my_panel.alpha < 100)
{
my_panel.alpha += 20*time_step;
wait(1);
}
my_panel.transparent = OFF;
It looks like it is doing something similar to what you are wanting.
