Given the value of the rest of the film being in black and white, other cinematic devices would have been very problematic to incorporate to illustrate a very pivotal character moment in the storytelling without interrupting the flow of the film or belabouring the issue.
What alternative would you have used.
Point of order: the murder of the child is a dramatic moment but neither the moment nor the character is pivotal (she isn't a character, either, just a cypher).
Many directors of fine black and white films have managed to dramatise moments like this without parlour tricks.
Also unnecessary - and actually mendacious - was having Schindler break down at the end and sob about all the people he hadn't been able to help. In truth, Schindler just nodded stiffly to Itzhak Stern, got in the car and left. That would also have been dramatic to shoot but honest.
Nobody really knows why Schindler did what he did. Even Itzhak Stern can't tell you. Opinions among the Schindlerjuden are divided and some of them hate him. Knowing that, Schindler's final scene in the film is worse than fraudulent (and the implicit blessing of the stone-laying documentary scene even more dishonest).