The visual system decomposes light entering the eye into an achromatic and a chromatic signal. Knowing whether this decomposition is additive or multiplicative is still a current research domain. Luminance has been found to be additive when measured physiologically but multiplicative in appearance. Demosaicing multispectral single shoot images show how additive decomposition is a linear solution to the inverse problem of mosaicing (sampling through a color/spectral filter array). But for reflectance estimation, a multiplicative decomposition would be preferred. I will show that these two decompositions imply two different geometries that share their vector spaces but not their metric.