Which colour can be made by mixing white and black poster colours?

Correct answer: Grey

The governing colour-mixing principle is that black and white produce neutral grey. The proportions change its value: more white gives a lighter grey, while more black gives a darker grey. Therefore option A is correct. Green generally requires blue and yellow, orange requires red and yellow, and violet requires red and blue; none of those hues can be obtained using only black and white poster colours.