In your image, the bare pcb carrier material looks dark, the traces (connections) look light green. That is because the copper underneath the solder mask (green lacquer) reflects the incoming light, making these parts brighter than the less reflecting carrier material.
A ground plane is a conductor like any other, but typically in the form of large areas, not thin traces. In your image, however, you see multiple planes. Obviously these are used to carry high currents. Which of these is the ground plane? The one that connects to ground (GND, 0 V), of course
. Likely, but not necessarily, the one I marked here: