In addition to sending a series of taps or you heartbeat, Digital Touch on the Apple Watch makes it possible to send disappearing images to fellow Watch owners using a series of colors. But the standard colors are kind of, well, boring. Thankfully, Apple has included a way to access a color picker when your selecting a color, as pointed out by John Gruber on Twitter.
Changing between stock colors is as simple as tapping on the small, colored dot in top right corner and tapping on the new color. To customize a color, long-press on a color to bring up a color picker.
Slide the circle around until you find the color you want, then tap Done.
The color you originally selected will be replaced with the new, custom color for future use. The only color you can't change is the middle color, white.