Flashing Yellow Walkie Talkie Icon
The Apple Watch Status Icons Manual Page mentions that the yellow icon is the Walkie Talkie app and suggests pressing it, BUT...
If you see the yellow icon briefly when viewing the watch, it cannot be pressed! It probably means you sent a Walkie Talkie invite which has not been accepted yet. Switching to the Walkie Talkie complication will confirm this, and offer to remove the invite (and the little yellow flash of the Walkie Talkie complication.