Gold nanoparticles (AuNPS) are known for their ruby-red colour related to their localized plasmon resonance and could be used for creating a new type of colour pigment. Moreover, the colour varies if the nanoparticle suspension is observed in transmission or diffuse reflectance spectra. We have systematically studied this two-colour effect, called the bichromatic effect, for twelve samples of nanoparticles in water with increasing diameters from 16 to 108 nm. The transmission and diffuse reflectance spectra were recorded and compared to an analytical model based on the scattering cross sections calculated using the Mie theory. We have also calculated and represented the colours within the CIE colour diagram. This approach provides a very efficient way for predicting the colouring power of plasmonic pigments when AuNPs are embedded in a material. It stresses that spherical gold nanoparticles may exhibit a good bichromatic effect when their diameter is between 70 and 90 nm.