[Mrs. Don-o]

Black plastic resists UV, the white plastic does not. That's why outdoor PVC items are often made of black PVC.

We use milk jogs made of white PVC to store rainwater outside. They start getting brittle, breaking down, and biodegrading in about a year.

As well, the black balls minimize shininess/bright reflective surface of water, which is what attracts birds and wildlife.

[beelzepug]

Remember when they raised that sunken ship from the Suez Canal by pumping the holds full of ping-pong balls?