Which habit is useful for preserving brightness in watercolour?

Correct answer: Mix a limited number of clean colours with minimal stirring

The governing concept is that transparent watercolour remains bright when its pigments and working water stay clean. Mixing a small number of clean colours gently avoids excessive pigment contamination and preserves luminosity. Thus B is correct. Repeatedly combining many colours can create a dull, muddy mixture; adding black generally lowers brightness, dirty water contaminates washes, and rubbing a wet surface can disturb the paper and weaken the painted layer.