OMG some freaky stuff just happened...need your advice?

My cat is black all over, some stuff got caught in his fur and I had to cut some small patches out with scissors, but so shocked to find that its only the TIPS of his fur that are black, when I cut about 5mm from his fur its WHITE as a sheet of paper underneath!!! How can this be possible, like imagine you are a barber and a guy with black hair comes in for a trim and walks out 5 minutes later with white hair?
If I gave my cat a haircut he would turn from 100% black to maybe 100% white instantly, is there some kinda weird freaky genetic stuff goin on there or what, or is it just normal for some cats?