A beta version of iOS 11.3 hinted that Apple would have a special pride face for the Apple Watch, and the release of iOS 11.4 and watchOS 4.3.1 has confirmed it, as reported by 9to5Mac.
According to code found in iOS by 9to5Mac, the watch face was inspired by the rainbow flag and will move if you tap the display. 9to5Mac also found video assets within watchOS which show the bands of color moving as the watch itself is moved, and the movements should be different every time.
The new pride face will be available Monday, June 4th at 12PM PST, meaning it will likely be announced and made available during WWDC. You could technically test out the watch face before Monday by changing the time on your Apple Watch, but there is the risk of losing data. If you can’t wait and want to forge ahead, go to Settings, then General, then Date & Time. Turn off the automatic option and then choose any date and time after June 4th, 12PM PST.