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?

Take a close look at the black ''PVC''. I think you will be able to read ABS on it. ABS is a different type of plastic. It used to be common, but am not sure it is available any more. It requires a different type of cement for the joints. However, there is a universal cement that can be used to join the 2 types. You shouldn't have any problems if whoever did the work mixing the 2 used the right cement. If you are planing any work, stick with the white stuff. If you must connect with the black, find some of the universal cement. I wouldn't count on the home centers to have it. You may have to go to a plumbing supply house.

If you have any more plumbing questions, it would be best to post them to the plumbing forum. There is a sharp guy there named Tom, knows more about plumbing than all the rest of us here put together. He may not see questions under home maintenance. I am just passing on what I learned from him about black drain pipe.

"I am just passing on what I learned from him about black drain pipe."
And passing it on very well I might add, Labman.
Let me add a little more to labmans answer.

There are four types of plastic in general use. Black plastic, which is ABS,(Acylonitrile Butadine Styrene) and white plastic which is PVC and CPVC,(Polyvinyl Chloride and Chlornated Polyvinyl Chloride) ABS is used for drainage. PVC is used outside the house for cold water feed from the pump or the meter Also drainage both inside and outside the house. CPVC is rated for both hot and cold lines and can be used to plumb a house on the inside and then we have Polybutylene pipe,(PB or Polly Pipe) which is flexible tubing on water piping.
And now you know about plastic pipoing. We switched from ABS to PVC in my area in the 70's because PVC was a stronger plastic. This doesn't mean that ABS should be torn out and replaced but that only PVC was a better product.
While earlier plumbing codes forbid the commingling of PVC and ABS, When Oatey came on the market with a all purpose glue that restriction was relaxed. If you have no leaks or complaints with your present set up then I can see nor reason for you to be concerned. Good luck, Tom