What is the most important precaution when mixing equal amounts of primary colours?

Correct answer: Measure exactly equal amounts of each colour

The governing concept is proportion in colour mixing. If a mixture must contain equal amounts, each selected primary colour needs the same measured quantity; otherwise the resulting hue and strength may shift toward the colour used in greater quantity. Thus option B is correct. Separate papers prevent proper mixing, a dry brush is not the essential condition, and adding white changes the value or lightness rather than preserving an equal primary-colour mixture.