I have marked my balls with a red blob on the T of Titleist ( both sides). Today in our group we had a guy with red dots on the sides of his Titleist and another with red blobs on his Wilson. It is rare to have several players with red as their color. Black, blue and green are more common. We were aware of the potential issue.
With a few holes to go , the twosome with "red" each hit in the rough. Player one hit a shot to the green. At that point the other "red" player figured out his cart partner had hit his ball. Frankly it is hard to understand how one could hit the wrong ball, knowing one needed to be careful. On the other hand, our 4th said he was amazed at how frequently the player whose ball was hit seemed to be involved in "wrong ball " issues. It is almost like he knows the ball is his and says nothing.
The player hitting the wrong ball was really upset with the other player. He removed his bag from the cart and walked the final 2 holes.
I felt hitting the wrong ball was his error, no matter how the other player may have contributed. My cart partner felt the error may have occurred because the other "red" was too uninvolved and by being passive may have created the issue. Kind of like watching someone tee off outside the teeing ground.
Sorry, I don't have more info. With the little we know, can you see a reason to assign blame to one or both?