Lets say a black dragon and a white dragon mate.
There is a 40% chance that any given egg would be a black dragon.
Another 40% chance that it is a white dragon.
The remaining 10% of the eggs are two-headed dragons. One white head, one black head and as for the body... well, I could be zebra-like, or all black, or all white, or even gray. I would go with a zebra-like pattern, though.
It might be taboo on their society since they could count it as blasphemy (their goddess is a multy-headed dragon, after all).
OK, since the question asked earlier seems to be poorly understood...I roll the d% and:
01 - 40 = White Dragon
41 - 80 = Black Dragon
81 - 90 = Two Headed Dragon
91 - 00 = WHAT?