How do I make an efficient lightbox popup?

Hi all,

I have some popups in my game and during popup is visible I would like to make the rest of the screen darker (lightbox effect). Now I have 2 ways to do in my mind:

1. Create black sprite in Construct and give it some opacity.

2. Create dedicated PNG (black with opacity) and use it in construct as it is.

As I'm aware opacity is resource consuming but is there any advantage in efficency in those 2 ways?

Thanks!