Since iOS 10.3, developers can set alternative icons which is very nice.
Is it supported in the companion watchOS app?
My app was recently rejected due to this.
Specifically, we noticed that for the user selected alternative icon themes no alternative Apple Watch icons matching the iPhone icon theme were submitted.
watchOS does not support alternate icon, as of watchOS 4.0 beta.
I ended up answering Apple Review team, the app is intended to not provide alternate icons in Watch App.