In which situation can red and blue produce a dull violet instead of a bright violet?

Correct answer: Red has a yellow bias and blue has a green bias

The key concept is unwanted pigment bias in subtractive mixing. Violet needs red and blue with minimal yellow or green contamination. If the red leans toward yellow and the blue leans toward green, those extra components introduce competing colour absorption and reduce saturation, producing a muddy violet. Thus option B is correct; white mainly lightens, and water changes dilution or transparency.