Which ability is improved by practising different ratios of secondary-colour mixtures?

Correct answer: Understanding colour bias and control

Changing the ratio of two paints changes the character of the resulting colour. For example, adding more red to an orange mixture makes it redder, while adding more yellow makes it yellower. Repeated practice therefore develops awareness of colour bias and improves control over mixtures, making option A correct. The other choices describe unrelated physical or naming actions.