Isn't one of the top reasons that we can CCW, following defending ourselves and loved ones, is making society a safer place? That means indirectly safer, simply by the BGs not knowing who is CCW and who is not. That also means intervening in crimes when we safely can. Seems like this is a situation where we safely can intervene.
From "Lock Stock and Two Smoking Barrels"
"Your gun says "Replica" down the side, whereas mine says Desert Eagle Five point O."
If the gun were clearly fake as the question states:
Yes I would intervene. The law doesn't recognize the difference between a real or fake gun in the commission of a crime and nor do I. It is the perceived threat of deadly force. In my state and most states you have the right to use lethal force in the defense of yourself or a 3rd party from the use of lethal force, including perceived lethal force (aka a fake gun). I would likely have a gun myself. This scenario, where a criminal is in the process of the commision of a violent felony, I would quitely draw, take aim, watch my background, and shoot the perp in the com and head several times. Makes it alot less expensive for tax payers (loss of money from the bank, police work, trial, incarceration). You've probably just saved tax payers $500,000 in the "due process" of this criminal. If I didn't have a clear shot and could not move to a clear shot, I would hold this man at gun point for the police. If I didn't have a gun or knife, I think a solid sucker punch or judo takedown, legsweep, or maybe a running shoulder tackle would take him out of commission quite quickly, and I imagine that I'd be supported by the other bank patrons (ala post 9/11 mentality). No, I"m not a mall ninja, but I am a strong, athletic, 6'1" man with quite a bit of martial arts and firearms training. Am I a Navy Seal? No. But I'm very confident that I could kick the average person's A**, especially with the aid of a surprise attack.
Sure there are people who get paid for this sort of thing but:
1. We all pay when bank crooks win
2. Who knows, you could be saving a life down the road (say the perp runs over someone during his flight)
3. Why not stop something you can stop? Not a real big risk
4. I can't imagine you'd be criminalized; quite the contrary you might be considered a hero. In fact, most banks offer $10,000 reward for information leading to the prevention or arrest of bank robbers. I would make a very compelling case that my intervention in a robbery was worthy of that reward.
5. 15 minutes of fame anyone? Could be worthy of its weight in gold. Not to mention a positive news article for CCWs out there, right. Besides, nobody needs to know that you knew the bank robber had a replica gun, right?
As far as painting the tips of REAL guns orange, what would the incentive be? Noncompliance because people think that gun is fake? Crooks want immediate compliance, not questions about the authenticity of their weapons. If anything, they would remove the plastic tips from their guns...