Change skin tone of emoji on iWatch

This might help....Tap the emoji button (looks like a smiling face). Swipe left on the standard emojiscreen to find the animated faces emoji. Using the Digital Crown, scroll up and down to move between the different options of your chosen emoji style. Use Force Touch to change the color to red.

While not an Apple Support Article, this is really great: https://www.imore.com/how-send-animated-and-standard-emoji-apple-watch

Glad it helped.