I know its been a while since this was posted but wanted to give you more details now that I saw it (bad habit of not reading the breeding section much).
So if an offspring is both Zero and Witblit that would be a Wero, but both Zero and Witblit only visually show if they receive the gene from both parents.
In the pairing you mentioned ( Het Witblits x Zero ) All offspring would be Het Zero, but with Het Witblit its a 50/50 chance of passing on the gene and the offspring being Het Witblit. You wouldnt be able to know for sure if each offspring is Het Witblit unless paired with a Het Witblit and some of their offspring are Witblit (having received the gene from both parents).
So for that pairing you would get Het Zero possibly Het Witblit.