I think you may be right about the whiskers coming in black and turning white but I can't say for sure. My friend's cat is about 17 years old now and he used to have black whiskers but now they are all white. Personally I think it's a lot like human hair. As we age, our hair turns gray/white, and I think the color of the whiskers could be a clue to the age of a cat.
Most of Nikita's whiskers start out black and then turn white. Don't know why that happens really.
Edit: For clarification, they are black at the root and then white at the tip. I.e two colours in the same whisker at the same time, they don't change colour from all black to all white.