We gave each ball different physical properties. Yellow balls are light and have lots of air drag. Green balls are massive. Red balls are very bouncy. This allows physical sorters to be designed.
Ah, I was confused by this as well, this information is missing from the article. I couldn't find any explanation about the colors in the application either.
Oh neat! Thanks for the reply to clarify that - I like having different properties as a concept added to the machine!