In 3" pvc sewer piping

What is the difference between the white and the black?
Can you mix the two in the same sewer run?

My house is 30 or so years old, I have both white and black in the same run (white horizontal and black vertical) it was that way from the day I bought it some 8 years ago - but I thought I heard some time back that you can't mix the two?

What are the problems if any?