Why can blues with the same name from different manufacturers produce different violets?

Correct answer: Pigments may have different red or green biases

The governing concept is pigment bias, sometimes called a colour temperature tendency. Two paints labelled blue may contain different pigments: one can lean slightly toward red and another toward green. When each is mixed with red, those hidden biases affect the cleanliness and appearance of the violet. Therefore C is the best answer. Value differences may matter in appearance, but palette size and brush length do not explain the pigment interaction.