A dynamic change in the character animation would be nice for indicating stamina, but if even the most basic animations still need help after all that MoCap, we know that it's a distant reality.
UI bars are bad. Because you have to look away from combat to read it. But a very simple, ambiguous "light" flashing faintly on the perimeters of the screen, or underneath our character, could indicate our stamina level without making us glance away. It would be seen even from a PERIPHERAL view.
So instead of a bar with 3 colors, you could have NO BAR, and instead have a larger color spectrum to indicate the precise degree of stamina within one of the 3 stages. Darker green would mean a healthier stamina, then it would fade to light green, then the light could "flash" dark yellow, meaning you've entered the middle stage, and it would get increasingly more orange in color until it flashed red.