Why can equal amounts of red and yellow from different brands produce different oranges?

Correct answer: Pigment transparency and colour bias may differ

The governing concept is the difference between ideal colour theory and real pigment materials. Two brands may use pigments with different transparency, concentration, undertones or colour bias, so equal measured amounts do not reflect identical spectral behaviour. Their mixtures can therefore produce different oranges, making option A correct. The colour wheel and the basic principle of secondary colours do not change from brand to brand.