The vehicle hardens with time. All light that leaves is additively mixing. Repeated transmission leads to saturation. Review Reflectance curve quantitatively defines the light that comes off a reflecting body. Compose by multiplication.
Light source can modify the color.
Water colors and printer's inks are designed to be work according to the simple subtractive rules. Artists' primaries Color seen from paint is result of many different types of reflection, abosorption. Bth subtractive and additive mixing are involved.