## 3 Results and Discussion  
The evolution of CIELAB coordinates of each of the considered color patches are listed in the Tables A1-A4. It should be noted that the white coordinate, corresponding to the ink-free spot of the color chart, undergoes a stronger variation in comparison to the other colors under study in all cases. This degradation, seen as a progressive shift of values, is especially remarkable in the b* coordinate. This implies a process of yellowing, extremely pronounced for the Glasspack (Table A4). The inks act as a \"shield\" for the substrate material, given their greater stability and less intense degradation. The most stable of the materials considering its low variation is the acid-free laminated paper (Table A1). Based on those tables, the CIEDE2000 differences for each one of the four materials under study are shown graphically (Figure 3). In general, the color patches with inks printed upon them show a relatively slow degrading, with a barely steep slate, staying in most cases under the \(\Delta E_{00}\) CIEDE2000 \(<1.5\) threshold, implying a minimal perceivable color difference, only noticeable by trained eyes [25; 26; 27]. This indicates good stability and robustness in the considered exposition period. Considering that materials such as papers usually are not as resistant towards chromatic deviation as other, more expensive materials like ceramics, it is promising to observe how small the induced error can be for the considered application. It is interesting to note that on half of the tested materials, the color patches with only one ink printed (black, yellow, cyan or magenta) show a subtly stronger degradation than the other colors, which need more than one ink to be printed, following the additive YCMK system. This reinforces the idea of the nature of the inks as protective covers of the substrate material. Therefore, the white ink-free patch shows a stronger degradation compared to the rest in the described graphs. Furthermore, its degradation can exceed a CIEDE2000 difference of value 3 for the acid-free paper and the Glasspack. It can be stated that the degradation of the ink-free spots is especially necessary to consider for the whole experiment (Figure 5), since an excessive difference with the zero-hour original value may lead to an erroneous color calibration process. This fact could compromise the whole monitoring mechanism. Figure 4: The six pictures from six different models of commercial cellphones that imitate the effects of image disparity by crowdsourcing. Therefore, a deeper observation of the white spot for all the considered materials has been done. Several \"dummy\" color charts have been generated. The colored spots have been conserved, but the value corresponding to white patch has been substituted in each one of the materials with the RGB color coordinates corresponding to the CIELAB of every taken measure for the given time checkpoints. This is done for all four substrates. Then, the calibration process as described in [19] has been run on each of them with respect to the corresponding zero-hour color chart as the reference. Thus, the drifting effect of the decoloring towards the yellow of the white spot on the whole calibration process can be measured. The subjects of calibration have been seven color stripes, in order to assess its effect on a variety of surfaces. Four of them represent silver and copper extracted textures from other photographs, since the indicator material coupons for the monitoring system are will be made of both metals. The three others represent a neutral grey hue (RGB = 128, 128, 128), a uniform copper color (RGB = 199, 140, 98) and a bright yellow (RGB = 255, 230, 128). They are used to see the effects of calibration with an erroneous white reference on different zones of the color space: the center, the region of copper tones, considering silver will mainly engulf luminance-predominant hues, and the yellow stripe represents the yellowing shown by aged whites (Figure 6).