The latest update for the iOS version of the YouTube app adds new streaming quality options, including HDR support for compatible clips.
While users can manually select HDR quality, the Auto setting will automatically enable the option if it is supported by both the video and device.
HDR (High Dynamic Range) supports a wider range of colours and deeper contrast levels than standard video, and is only compatible with supported displays.
Apple's previous-generation iPhone X also includes HDR support, as do a number of Android smartphones.
It should be noted that 4K video content is still unavailable on YouTube for iPhones, a problem which may be attributed to Apple's lack of support for Google's VP9 Codec.