What is directly affected by changing the proportions of primary colours?

Correct answer: The hue of the mixed colour

The governing concept is proportion in colour mixing. If the relative amount of one primary pigment increases, the resulting mixture shifts toward that colour; for example, adding more red to a red–yellow mixture makes the orange appear more reddish. The paper length, brush size, and subject remain physical or planning choices and are not changed by the pigment ratio. Therefore option A, the hue of the mixed colour, is the direct effect.