Apple's Digital Touch feature on the Apple Watch lets you send your heartbeat, draw photos or tap on the wrist of a friend who also has an Apple Watch. When the Apple Watch was first released, Digital Touch was limited to sketches consisting of one color of digital ink. When drawing on your watch, if you tried to switch colors, the sketch would be sent and you'd be forced to start all over; with Watch OS 2, that's no longer the case.
You can now sketch and craft temporary drawings for your Apple Watch-toting friends using as many colors as you'd like. You can add another color to the drawing by tapping on the color selector when drawing. Don't feel rushed to pick another color, take as long as you need to adjust the shades (long press on a color icon to customize) and then go back to drawing. Only after you've stopped doodling will your Apple Watch send your carefully crafted masterpiece.
If a friend hasn't yet updated to Watch OS 2, your handiwork will be wasted, as the color you started with will be the color used for the sketch.