nouns - Which is correct: "A contact between green and red balls/a green and a red balls/a green and a red ball"?

I'm writing a technical paper, where I discuss the behavior of contacts between two slightly different materials. What is the correct way to write it:

1. A contact between a green and a red balls




2. A contact between green and red balls




3. A contact between a green and a red ball

It sounds very simple, but I couldn't find a clear cut answer in any of my text books. I need to make it clear to the reader that I'm talking about a single contact between one red-type ball and one green-type ball.

Many thanks for your help!