Atticus says at the trial in To Kill a Mockingbird that "this case is as simple as black and white." What does he mean?

Atticus has known since the day he took the case that the
defense of Tom Robinson was not winnable. He tells his brother Jack (in Chapter 9)
that

"The jury
couldn't possibly be expected to take Tom Robinson's word agains the
Ewells'--"

The case boils
down to Tom's word against that of Mayella and Bob Ewell. Although the Ewells are the
"disgrace of Maycomb," they are white and Tom is black, and in 1930s Alabama, the word
of a white person is always believed over that of a black
person.

There is actually a double meaning in Atticus'
"black and white" statement. In one sense, he means that the case is "cut and dried":
The facts seem obvious. Atticus has seemingly proved that Mayella's attacker must have
been left-handed (as is Bob Ewell), and Tom's left arm is crippled. Bob and
Mayella's testimony conflicts with one another, and Mayella changed her story several
times while on the witness stand. Tom seems to tell his story truthfully and his
testimony seems plausible; the Ewells' testimony does not. But, as Atticus knows, the
trial all boils down to a white woman charging a black man with rape, and even Atticus
can't convince this jury to be colorblind in this case.