The patent describes a band with electrochromic elements, that is a material that can change color and opacity in response to small electric currents.
The most familiar example in everyday life is auto-dimming rear-view mirrors in cars. A light sensor detects when you are being dazzled from behind, and activates an electric current that reduces the opacity of a film or glass layer covering the mirror.
Another example you may have seen is in glass-walled conference rooms, where the glass can be instantly switched between transparent and opaque modes. This makes for a much neater and faster alternative to blinds.
This same technology can be used to display different colors in a material. By selectively activating different electrochromic sections within a band, it can be controlled in a similar manner to a dot-matrix display, to show symbols, letters, or numbers.
Color-changing Apple Watch band
Apple’s patent (spotted by Patently Apple) illustrates the idea with a three-stripe design, in which the color of each stripe can be independently controlled. That would allow both solid colors and stripe patterns to be displayed.
Apple would achieve this by interweaving electrochromic elements into a fabric.
The patent describes how you could, for example, adjust the color of the band to coordinate with your clothing.
The patent that relates to a fabric Apple Watch band that includes an electrochromic feature allowing color-adjustable elements that could change the color scheme and design in 3 distinct band zones to give the watch band a customizable look unique to the user […]
Users may desire the ability to customize their watch bands to express variety and style. For example, a user may desire a watch band of a particular color based on the user’s selection of clothing, other wearable articles, environment, or another preference.
Apple notes that people can do this already, by physically changing the band, but that this invention would remove the need to own a variety of bands, and saves the effort of swapping them.
Controlled by an app
An illustration shows an Apple Watch app being used to select the colors and styles, and this then being activated in the band.
The patent goes on to describe changing the color of the band to indicate notifications, and even displaying content on the band like symbols and words.