Hello Jay,
The transparent effect is easier to achieve than most people think.
At first, paint continuously as if there's NO transparent layer, using the background colour. The transparent layer is gradually built up by adding zinc white to the background colour and adding layer after layer until you're satisfied with the overall effect.
Of course, in cases like these (where the figures are based on original paintings), the same original painting will give you an excellent example of what should be achieved.
It's a great opportunity to study and emulate the techniques of the master and I admit that I've learned quite a few tricks while doing the Van Der Weyden flatz.
Cheers,
Quang