How ride efficiency is calculated? (The red glow around the range number)

Ride efficiency is calculated as energy spent per distance travelled(for the last 10 sec).

Hence, even if you go slow uphill, there is a certain amount of energy that will have to be spent just to gain that potential energy. Hence the red glow.

The same thing applies while coming downhill, the energy consumption will be low as most of the energy spent is coming from the potential energy and not the battery pack.