Why can excessive brushing during wet-on-wet acrylic blending make colours muddy?

Correct answer: Different colours repeatedly mix and lose clarity

The governing concept is controlled wet blending. When wet layers are brushed repeatedly, pigments from neighbouring colours are continually redistributed and mixed. Instead of retaining clean colour areas and a deliberate transition, the mixture becomes optically dull or muddy. Limited strokes, appropriate moisture, and a clean brush preserve clarity. The brush does not simply dry the paint, the canvas does not turn it black, and water cannot become metallic; therefore C is correct.