With every new Android version, we get a long list of useful new features and a few, well, quirky ones. Nevertheless, we're here to dissect every aspect of Lollipop and that includes this fun new feature of the Clock app. Short story: the app's background now switches gradually throughout the day to mirror the time. Just in case, you know, looking out the window or reading the actual time isn't clue enough as to what it feels like, outside.
Long story? There are purple hues for the early morning and late evening, blue for mid-day, and darker shades of blue and indigo in between. At night, a dark grey background should don your clock. A typical day's colors have been sequenced below to give you an idea of the color succession. It's very Material and, despite not being very useful, it shows the extent of the attention to detail in Lollipop.