YouTube doesn't control what any specific device does with the image. Nor can we, actually. Even if the device uses the appropriate display transform for the media being played, different OS and app settings, hardware settings, hardware, and even the ambient lighting while viewing, all have a major factor in the image as we visually recieve it. So the apps can be using different options on the same device ... pretty common. Colorists are taught this maxim to begin with ... some variation on "You can't fix gramma's green TV." And it is really a thing. One network level colorist I was reading about noted that he'd done a really cool car commercial one fall, really shot spectacularly, color was just a kick to work with. He was really proud of it. Went to his yes, grammas' at Christmas to visit. Her TV was way magenta I think it was ... and he was horrified at what her TV did to his beautifully graded commercial when it came up. But his gramma was used to everything being off to that hue, so it was normal to her, and she thought the commercial really very nice. She didn't see the color as being 'off', it was just "normal". Probably that TV had been a bit magenta to start with, and went more so over the years. Typically going green was more common, but the CRTs could go pretty much any direction. It gets back to two things: first, the Rec.709 specs require that screens do a specific thing to display the image ... that transform that applies a fairly strong contrast curve in the shadows on display ... it's not 'cooked' into the file. So if the device or an app does something different, you get a different image. Second, it's also a factor of what the the device and screen are set and/or capable of doing. And that also is gauranteed to vary. As has been demonstrated several times by color scientists and high-end colorists, you can have two "identical" Grade 1 Reference Monitors sitting side by side. Calibrated and profiled (the profile checks the image to see if the calibration was correct) to identical specs ... and fed the identical image from a breakout device/box. And to the people sitting in the room looking at the two monitors, they will not be the same image. One person will see say part of a screen going slightlly magenta, to another viewer, the same part of the same screen is tending slightly green. That's on one of the two screens. The two screens will visually be slightly different, even though technically they test identical on very high-end calibration gear.
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