Two colours mixed on a wet surface and formed an unwanted third colour. What is the correct way to prevent this?

Correct answer: Test the colour compatibility first and limit the area where they meet

The governing concept is managing pigment interaction in wet-on-wet watercolour. On a wet surface, pigments spread into one another, so colours that combine poorly may produce a dull or muddy third colour. Testing the pair on scrap paper first and limiting their contact gives better control. Adding water or wetting the paper further increases spreading, and applying all colours together removes control. Therefore, option A is the correct preventive method.