If you don't use the same color management, which has dramatically shifted in Pr25.x, you've got a problem. And if one is on a Mac without Reference modes, the other on a Mac with Reference modes and ALL other systems .... you have a separate problem.
First, for iPhone media in general, neither of you were doing what should be done in V25 and forward.
Lumetri SETTINGS tab, the one NAMED Settings
- Display color management, auto detect log, auto tonemapping ON; extended dynamic range on if on a Mac;
- Sequence to Rec.709, as even HLG "out in the Wild" is still the Wild Wild West as HDR is nutso still.
- Set your working space to what you prefer, as this only sets the space the color correction tools work within;
- Use only export presets for Rec.709 media, which do not have HLG or PQ in the preset name.
Viewing Gamma and Monitor Issue
The long used and professionally required display transform for Rec.709 video is essentially gamma 2.4.
That is used on nearly everything ...including Macs with Reference modes set to HDTV ... except ...
Macs without Reference modes use a display transform of essentially gamma 1.96 for Rec.709 video media on their monitors.
Do you see the problem there? You cannot display an image with two separate display transforms with large differences in the shadow regions and get the same image on screen.
But ... even Macs without Reference modes can get a sample display of the difference. Have them look at the same video clip QuickTime Player, Chrome and/or Safari, then in VLC and PotPlayer, and YouTube/Vimeo while using Firefox browser. And compare the image most other users will see versus what they've seen in QuickTime Player.
So as to your situation, and for many editors, the next bit is crucial.
What is your client's Mac, do they understand why there is a 'gamma' difference, and what do they want to do about it?
That is the often thorny other issue you face. Many Mac users insist that their Mac is essentially perfect, and shows the proper image ... and will not hear about any differences with other stupid WRONG non-Mac junk. They refuse to check with VLC/Potplayer/Firefox.
And some simply state "but those are not properly color managed apps". But that is only "color managed" by the OS ... which is more color-mangled than managed.
Those apps actually use the correct display transform on more systems than QuickTime Player.
Unfortunately, the editors who are rather Mac-centric often don't realize that they are in a smallish screen minority. But if that is the client, well ... they pays the bills, they sets the rules.
If they are wiser, and listen to professional advice, you may get a very different set of rules. But you have to bring this up and see what they require you to do.
If they require you to work to the lighter Mac-without-Reference gamma, set your rig's Viewing Gamma to 1.96/Quicktime, while working for them. And send them their stuff, and IF they set their Premiere color management as I suggest above, everying will be as they want.
For your general work, for others ... as I doubt you are in that nearly-blackened room colorists work in ... the normal "bright room" gamma for Rec.709 video is actually, by the proper standard! ... the gamma 2.2/Web option.
Not because it's for the Web ... which is a misnomer. Nearly all computer monitors actually use gamma 2.4 for Rec.709 video. Gamma 2.2 for sRGB media is a stills thing, NOT a Rec.709 video thing. And most of the monitors I've been around are set to work accordingly with Rec.709 media when left on their auto settings for such things.
ONLY use the gamma 2.4/broadcast Viewing Gamma setting when you are in a darkened room while working.
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