White looks gray on iphone

I work for/with/teach pro colorists. You're misusing something if that output is HDR. But it's easy to see how this happens.

In HDR workflows, the HDR part is specular highlights. The working brightness for "graphics white" is given as essentially that expected for a white paper in daylight, and typically the 203 nit option is the most used in professional grading.

If the graphic white was as bright as the brightest pixel of the show, say 1,000 nits ... anytime you had one pop on screen you'd about blind the viewers. Understand?

Graphics are in most streaming set around 200-205 nits.

That is why that "white" that you added is "gray" ... it's a graphic item. Premiere is handling it precisely how graphic items are expected to be handled in HDR workflows.

So how did you create that background? And ... should you be using an alpha channel process, perhaps?

Thanks, I will check this out! 

It's interesting that premiere pro can't create an actual white surface. The rest of the video looks fine. 

That sounds like you are creating or exporting an HDR video, perhaps unintentionally. Double check your Sequence Settings and verify that it's set to Rec.709. Then from there the MP4 export settings should also be the same and you should have no issues.

Correction: The white layer of premiere pro (#fff) looks gray on the smartphone screen.