## 4 Results and Discussion
### Fade Analysis of the Pigment Color Blocks  
1 Lead Red: In the simulated aging fading, from Figures 10a, 11a, and 12a, the lightness (L) decreased from 64.5 to 57.3, the chroma (C) decreased from 75.1 to 57.8, and the hue (h) decreased during 20-250 h, increased during 250-370 h, and gradually decreased from 570 h. This analysis shows the following: As shown in Figure 7, the experimental light source did not show a spectral response in the longer wavelength range; its spectral response was mainly located in the 290-450 nm shorter wavelength range, which had a higher spectrum amplitude energy and was more easily excited. Wavelengths primarily in the range of 610-700 nm corresponded to lead red excitation; lead read is mainly composed of Pb\({}_{3}\)O\({}_{4}\). Under long-term constant temperature and humidity conditions, high-energy blue-green light was absorbed to generate PbO\({}_{2}\), resulting in a decrease in lightness and chroma. Additionally, an overall yellowing and darkening occurred in the gelatin due to its absorption of high-energy spectra. However, the increase in hue between 250 and 370 h could be attributed to the decomposition of gelatin resulting from the absorption of a large amount of high-energy blue-green light. After 570 h, an overall shift toward a dark red hue occurred. 2 Lithargite: In the simulated aging fading, as can be observed from Figure 11b, a nearly linear decrease in lightness (L) with a magnitude of 10 was observed. Lithargite underwent a transformation from PbO to PbO\({}_{2}\) with formation of a small amount of PbCO\({}_{3}\cdot\)Pb(OH)\({}_{2}\)[7], resulting in an overall decrease in lightness. From Figures 10b and 12b, the hue (h) increased from 71.5 to 73 during 0-220 h, while the chroma (C) sharply decreased during the same period and exhibited a relatively slow decline after 220 h. A substantial amount of short-wavelength blue light was absorbed by both lithargite and gelatin during this period, as indicated by their significant reflectance in the wavelength range of 570-670 nm. As a result, the gelatin underwent decomposition, leading to a shift toward a blue-green hue in the overall chromaticity. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**PigmentModel** & **Lead Red** & **Lithargite** & **Rouge** & **Cinnabar** & **Azurite** & **Malachite** \\ \hline BPNN & \(\Delta\)E = 075 & \(\Delta\)E = 044 & \(\Delta\)E = 2.36 & \(\Delta\)E = 0.30 & \(\Delta\)E = 0.30 \\ \hline LSTM & \(\Delta\)E = 067 & \(\Delta\)E = 044 & \(\Delta\)E = 2.08 & \(\Delta\)E = 0.3  
3. Rouge: Rouge is a commonly used natural pigment with unstable properties that is prone to fading under prolonged exposure [4]. As shown in Figure 11c, a significant increase in lightness (L) with a value of 25.1 was exhibited by rouge during 400-600 h under simulated sunlight irradiation, resulting in an overall phenomenon of whitening and fading. From Figures 10c and 12c, a downward trend followed by a slight upward trend was observed in the hue (h), with a decrease from 24.8 to 3.9 and then an increase to 17.2. The overall hue gradually shifted from deep red to bluish-red. The chroma (C) demonstrated a decreasing trend with a value of 22.3. The experimental light source used in this study had a low spectral response in the range of 600-700 nm, leading to the prolonged absorption of high-energy blue-green light. Due to this long-term absorption, the red longer wavelength spectrum absorption was low, resulting in the aging and decomposition of gelatin, fading of rouge, and an overall lighter color. 4. Cinnabar: Cinnabar is also in the red family, but it is relatively stable. Its main component is mercury sulfide, a hexagonal crystal system, and its crystals are plate-like or rhombic [7]. When heated, a transformation of the crystal structure to a cubic (isometric) crystal system with a sphalerite-type structure occurred, resulting in dimming of the lightness. Upon cooling, the original structure was restored.