## 4 Discussion
Spectraval(tm) pigments do not produce color based on selective absorption as traditional pigments do.Moreover, opposite to the subtractive color model, the output is white instead of black when mixing red, green and blue inks. Previous works have addressed how these pigments allow printing additively [20]. However, the gamut of the Spectraval RGB pigments is much smaller than the gamut of sRGB or Adobe RGB (1998) color spaces, therefore the pigments are considered not suitable for realistic color prints. In this study, there are several variables to consider: color of the substrate, printing method, concentration of inks, combination of inks and order of deposition, and whether the image was produced from a positive or negative film. Images were used as positives for subtractive/absorbent inks deposited on white paper, or as negatives for reflective inks applied on black paper. The Amber Orange and Solar Gold pigments are both reflective and absorbing. To understand its behavior, Amber Orange was applied using a positive plate on black paper and as negative on white paper (S11 Figure A15 and OL 13 Figure A6) The prints made on black paper from positives show black areas where there should be white areas depicting the apparent glossiness of the amber beads. As expected, the variable thickness of the layer affects the saturation, chroma and intensity of the prints. Screen printing produces thicker layers than offset litho or direct litho. Screen printing also contributes to a higher chroma as exampled in S 16 (Figure A12) but not necessarily to a higher luminosity, as it depends on the kind of inks and substrates used. DL14 (Figure A5) and DL15 (Figure A2) have the highest luminosity. These were images printed on white paper using only Solar Gold ink and Solar Gold plus Magenta ink, respectively. The measurements on amber-colored areas (Figure 7) show that the luminosity is reduced by around 10% whilst the chroma remains almost the same (Figures 8 and 10). The chroma is increased in the red spots; when the magenta is under-laying the Solar Gold layer the hue is shifted towards the red. These images have a higher luminosity because they were printed on a highly reflective substrate and do not have a dense layer of absorbent pigments  
The images printed on white paper with process inks (CMYK) with the methods of direct (DL 2 Figure A4) and offset lithography (OL 1 Figure A3) also have a higher luminosity than the images printed on black, with the exception of S12 (Figure A14). As discussed above, screen printing results in thicker layers. The layer of ink is dense enough to reduce the luminosity, while increasing the chroma of specific colors. It would be expected that offset litho would give a higher luminosity, as the ink layer would be thinner than using direct litho. DL 2, however, was overprinted with Iridin(tm) Solar Gold resulting in a higher luminosity than using YMCK only. This effect proves the layer acts as a mirror as stated above. When comparing the chromaticity of DL 2 and OL 1, we see that it is considerably higher for OL1, which means that overprinting with Solar Gold, as in DL2, reduces the intensity of the color of amber and red beads. Observing the measurements of background--which are not altered by overprinting Solar Gold--wenotice that the luminosity is not considerably affected, whilst the chroma of DL 2 is higher (as the pigment layer is thicker). Among the prints on black paper using Spectraval(tm) inks, screen printing gives higher luminosity values than lithography as the ink layer is thicker (S5 Figure A1, S8 Figure A13, S10). There is a difference in the concentration of pigments used between these images; Figure A13 was made with half the amount of pigment compared to Figure A1, therefore its reflectivity is lower. The RGB print made by direct lithography (DL7 Figure A8 has a higher luminosity than the ones made by offset litho (OL 9 Figure A7, OL 6 Figure A10 and OL 3) also due to the layer thickness. The difference between OL 9, OL 6, OL3 is the number of passes.