The dark blue are the deep veins.

The light blue are the superficial veins.

See if you can find a connecting (or perforating) vein* between the superficial and deep veins.  (Although the artist did not show all of them, normally there are several in the thigh and more in the calf.)      

* Hint:  Look half-way between the words femoral and popliteal on the right side of the diagram.  The perforating vein is at right angles to the greater saphenous vein and is dark blue.