00:00Shoot
Turn on Log. Then forget about it.
Log is the iPhone keeping everything the sensor saw. It looks flat and grey on the screen. That is correct. The colour comes back in the next step, and you get to choose how much.
It lives in Apple's own Camera app. By default it switches itself off when you leave Camera, so make it stick: Settings › Camera › Preserve Settings › Apple ProRes on. After that, Log is your default.
CameraVideo⋯FormatProRes · Log
The badge at the top reads ProRes Log while it is on. No badge, no Log. iPhone 17 Pro records Apple Log 2, older Pros record Apple Log, and LutKit reads both. If your Camera app looks different, the same switch is in Settings › Camera › Formats.