Do you mean like window film? I would do white with a small repeating pattern. Definitely no black.
Non Adhesive Window Film, Static Privacy Glass Film, Frosted Window Cling, Glass Decoration/Removable/Heat Control/Anti UV/Vinyl for Office and Home Door Window Decoration,17.5In. By 78.7In. https://www.amazon.com/dp/B072FP1M4H/ref=cm_sw_r_cp_apa_i_pohgBb0THY17Z

Light frosted film...

why a film at all? if you're afraid of people looking in, do a beveled glass. can't really see through the storm door anyway. get a Ring doorbell camera.

Hi Susan. In my Guest Bedroom I used this 'window film' to hide the neighbor's rooftop Dish & A/C. I think a window film is much nicer than wallpaper on your door and sidelight, as it allows natural light into the hallway. It mimics etched glass very well. There are many many designs available. You can find sidelight sizes for a simpler application. Artscapes.com

LB..this post is over a year old!

Thanks. Yes, I recently spoke to Susan last week. I just thought I'd post it here for a window idea. We didn't discuss this dilemma though. I was on an older one of hers. We reconnected.