Nicely done, but...

You're forgetting that we're not talking about people in this series but of an, in fact, very different species than modern man.

Claiming that any aspect of that species must resemble modern humans' qualities is utter foolishness.

There is simply no way of knowing what the colour of their skin might have been, or even whether it was showing as much as was assumed on this series...

You can disagree with some of the assumptions, obviously, but in total it was a very nice piece of work, don't forget that.

The actual fact is personally I believe that the ergaster had more hair than depicted onscreen - the transitional creatures wouldn't be white or black, but in between. Look at their orangy brown skin on Walking With Cavemen.

Not exactly white is it?

If the chimp is indeed our closest living relative, then it is most likely that our most recent hair-covered relative had white skin under its hair. Which would mean that, yes, dark skin evolved from light skin, and light skin subsequently from darker skin. However skin color evolves much more quickly than other morphological changes, such as hair color or face shape. Darker skin wouldn't have existed while Africa was a jungle, or while humans were mainly tree-dwellers, as they would have had protection from the sun.