What should be done before mixing two poster colours?

Correct answer: Take small amounts of both colours

The governing concept is controlled colour mixing. Taking small amounts of both poster colours first allows the artist to test their proportion, observe the resulting shade and adjust it gradually. It also prevents unnecessary waste if the mixture is too dark, too light or otherwise unsuitable. More colour can be added later, whereas an excessive mixture is difficult to correct. Therefore, option A is correct; the other choices are wasteful, unrelated or technically unsound.