This is actually one of the most clever examples of rhetoric that Atticus employs in this brilliant novel. Note that he is presenting his summing up for the defence of Tom Robinson and he starts off by saying that "this case should never have come to trial," before going on to describe it as "simple as black and white." Of course, there is a massive play on words here. Firstly, Atticus is using a common phrase to point out how this case and the judgement that the jury should make is easy and that there is no doubt or uncertainty to suggest that Tom Robinson was not innocent.
However, and crucially given the overall theme of this novel, the reference to black and white is double-edged because of the way in which it explicitly refers to the discrimination and prejudice that has played such a part in Tom Robinson's arrest and trial. By saying that this issue is "simple," Atticus paradoxically points towards the tremendous complexity of race relations and also the very simple way in which stereotypes and pre-conceived notions of race have led to Tom Robinson's wrongful arrest.
In Chapter 20, Atticus begins his closing remarks by commenting, "This case is as simple as black and white" (Lee 124). Referring to something as "black and white" is a common saying that means that something is easily distinguished. Atticus's statement also employs a double meaning. Atticus is referencing the obvious racial aspects of the case. There is no significant evidence to convict Tom, and the case comes down to a white person's word against a black man's word. Atticus is defending an African American in front of a prejudiced jury and draws attention to the fact that the case hinges on the deliberate racial bias of the jurors. By commenting that the case is "simple," Atticus points out that there are no significant pieces of evidence that point to Tom Robinson's guilt, and the verdict comes down to the jurors' decision to act upon on their prejudice. Unfortunately, Tom is wrongly convicted and becomes another victim of racial injustice.