The red color is an integral part of the corporate identity of the YouTube service. The developers made many changes to the user interface, but the red elements often remained unchanged. But now YouTube is testing color variations for the play indicator in its Android app.
For a long time, the watched part of the video on Android, iOS and PC apps in the playback bar was red, and the unwatched part was a gray line. According to 9to5Google, on some of the Android devices, the app now has a white or gray color of the watched part of the video instead of red with dark mode enabled on the smartphone.
However, if you tap the progress bar and interact with the player controls to move between sections of the video, the color changes back to red. The red progress bar also returns when the dark mode on the smartphone is turned off. The rest of the YouTube interface has remained the same.
The iXBT.com editors can confirm that this innovation has already been implemented on one of the Android smartphones (Xiaomi 10T Pro), and everything is still unchanged on the iPhone 13 Pro Max.