Marked Wrong Ball on putting green

[Feyderlange]

Me and my playing opponent were both on the green, just 2-3 feet apart. I walked up to the green and marked 'my ball'. Unfortunately, we used the ball of same brand, his ball with a faint marking at one side which I couldn't identify when i marked the ball. When the opponent walked to the green, he assumed the other ball was his and marked it as well :)

He proceeded to putt and sank it into the hole! We applauded to his great putt. 

Then it was my turn. When I placed the ball and saw the unfamiliar marking, I knew I'm screwed. 

I didn't make the putt with his ball, but took over my ball from him, replace the ball from where he last putted. We both gladly took 2 penalty strokes each.

I unintentionally marked the wrong ball which inevitably made him putt the wrong ball. That said, I had not hit the wrong ball. What would u guys do, for discussion sake?

Cheers

[RickyBobby_PR]

Based on my understanding of the rules it would have been what you did.

Having played the wrong ball before in a friendly match I learned my lesson so I always check to make sure it’s my ball when I mark on the green or before hitting an approach

[cnosil]

3 hours ago, Feyderlange said:

Me and my playing opponent were both on the green, just 2-3 feet apart. I walked up to the green and marked 'my ball'. Unfortunately, we used the ball of same brand, his ball with a faint marking at one side which I couldn't identify when i marked the ball. When the opponent walked to the green, he assumed the other ball was his and marked it as well

He proceeded to putt and sank it into the hole! We applauded to his great putt. 

Then it was my turn. When I placed the ball and saw the unfamiliar marking, I knew I'm screwed. 

I didn't make the putt with his ball, but took over my ball from him, replace the ball from where he last putted. We both gladly took 2 penalty strokes each.

I unintentionally marked the wrong ball which inevitably made him putt the wrong ball. That said, I had not hit the wrong ball. What would u guys do, for discussion sake?

Cheers

My interpretation is that the marking of the ball wasn't a problem.   Per rule 9.5B Exception 2:  Marking and Lifting Player’s Ball on Putting Green by Mistake: There is no penalty when the opponent marks the spot of the player’s ball and lifts it on the putting green in the mistaken belief that it is the opponent’s own ball.

https://www.usga.org/content/usga/home-page/rules/rules-2019/rules-of-golf/rule-9.html

You could have avoided the penalty had you switched balls when you noticed the ball was not yours and putted with your ball.   Your opponent correctly took a penalty for playing an incorrect ball.   Both of you should have replayed your putts.   You should always verify your mark on the ball to ensure it is yours. 

[TR1PTIK]

Based on the information provided, did you not both in turn wind up playing from the wrong place? Depending on whether or not you'd define it as a "serious breach" or not would further determine what penalties apply. Of course if it's between two buddies on the course, then playing form the wrong place is of little consequence so long as you are both in agreement. At least in my opinion. However if you were in a tournament, playing from the wrong place without correction could result in disqualification as I understand it.

[DaveP043]

Use of the word "opponent" implies match play.  If thats the case, @cnosil has it right, no penalty to you for mistakenly marking and lifting your opponent's ball.  Then he played from the wrong place, using an improperly substituted ball.  He gets the general penalty, he loses the hole.

In stroke play, he played from the wrong place, using an improperly substituted ball.  But its a single act, so only a single general penalty would apply.  Based on the proximity, its unlikely to be a serious breach.  The OP, however, played his own ball from the correct place, and isn't penalized for unintentionally marking a ball not his own, no penalty at all.