In the Watch HIG, Apple recommends using black backgrounds for application UIs:
A black background blends seamlessly with the device bezel and maintains the illusion of no screen edges. Avoid bright background colors in your interface.
This doesn’t tell the entire story, though, because it’s likely that the Apple Watch uses an (AM)OLED display.
One of my first impressions of the [Apple Watch] user interface was that it used a lot of black. This makes the face of the device feel more expansive because you can’t see the edges. But more importantly, those black pixels are saving power and extending the life of the display. It’s rare that engineering and design goals can align so perfectly.
And from what we’ve seen so far of the watch, that black is really really black. We’ve become accustomed to blacks on LCD displays that aren’t really dark: that’s because the crystals that are blocking light let a small amount pass through. Total darkness lets the edgeless illusion work.
As with iOS on the IPS LCD displays of the iPhone and iPad, the Apple Watch uses the inherent qualities of the display technology to define the choices behind the UI. Black doesn’t show quite as well because, as Hockenberry says, a small amount of light still gets through. There’s no hit to power consumption when the display is entirely white because the backlight is always on, and the subpixels merely change the arrangement of the crystals to vary their brightness. That’s one reason why iOS is basically made of white.
On the flip side, subpixels of (AM)OLED displays are also its backlight, as it were. The colours used in the onscreen content determine how much power is being used; when white is used, it means all of the subpixels in the white area are turned on, which makes the display use far more power than it would outputting darker colours.1 Therefore, the Apple Watch’s UI is mostly black. (AM)OLED displays also typically display colours with far greater saturation than LCD displays, so Apple is also using a lot of very bright colours in the Watch UI.2
I’d love to see the Watch display under a microscope. Can’t wait until the Anandtech crew get their hands on one.
In hindsight, this also explains the greater use of bright colours in iOS, and Apple’s focus on making iPhone displays that have a full sRGB colour gamut to reproduce those colours as precisely as possible. Apple likes consistency. Sometimes. ↩︎