Best color conform conversion type

Apple ProRes 422 doesn't handle color the way you seem to believe, other than retaining the 10-bit, 4:2:2 color information of the video.

Its purpose is to shoot all of your original video in a lossless format. Read that as huge raw video files. If you shoot H.265 or similar, then the video has already had the lossy snot compressed out of it before you even start your editing. Then your footage gets a lossy compression again when you render the finished sequence to MPEG-4 or other lossy compressed format.

If you start with H.265 and output Apple ProRes 422, the only thing you gain is not performing another lossy recompression to the already lossy compressed H.265 video. It's like going from a medium quality JPEG to a lossless format like PNG. The image has already been permanently butchered by the JPEG compression. Saving that to a PNG only prevents it from getting worse. But in either case, you can't restore what has already been thrown out.

As far as color, the standard is Rec. 709 (or BT.709), which should be the color space already assigned to the video when it's shot.

ProRes 422 is *not* lossless. Apple's own ProRes whitepaper describes it as "A high-quality compressed codec."

H.264 does not have any one quality level. This varies widely depending on the GOP parameters and bit rate. In general, 10-bit 4:2:2 H.264 with a GOP size of 10 at 150-200 mbps is visually indistinguishable from 500 mbps ProRes 422.

This can be measured using Apple's free command-line tool AVQT. It is designed to measure the perceptual quality difference between two encodings of the same scene. It can be downloaded from the Apple Developer site, but that requires you to create a free developer account. See the WWDC21 talk on AVQT: "Evaluate videos with the Advanced Video Quality Tool"

On YouTube, if you search "How to Use Apple's Advanced Video Quality Tool," you'll find a tutorial on using AVQT.

You can also measure visual encoding quality using a tool developed by NetFlix called VMAF (Video Multi-Method Assessment Fusion). VMAF is built into certain versions of ffmpeg. On macOS, ffmpeg-VMAF is available through the Homebrew package manager.

H.264 is a very stable codec, we use it all the time in broadcast. We export H.264 with no visible image quality loss. Exporting ProRes 422 gains you nothing after that but eats up more file space.

H.264 is a very stable codec, we use it all the time in broadcast.

I didn't say anything about its stability. I said it's lossy. Well, I did only mention H.265, but H.264 is just an older version of the same process.

We export H.264 with no visible image quality loss.

Um, okay. But distributing the final, edited video for broadcast in this format would hardly be anything unusual. Broadcast OTA signals are also compressed, so the completed H.264 video gets a lossy compression applied to it again. Anyone expecting artifact free video in this medium will always be disappointed.

Can you see it on a 4K TV even when the video is lowest HD choice of 720? If you're sitting back at a normal viewing distance, not really. But get up close and you'll see lots of soft pixelization from the lossy compression everywhere.

Exporting ProRes 422 gains you nothing after that but eats up more file space.

Just as I said.

Not sure what your goal is. Can you be more specific? Not sure what "color conform conversion type" means.