## 3 Results and Discussion
### CIELAB Analysis  
Color recognition by the human eye can be separated into three elements: brightness (luminance), hue, and saturation. These are the main elements of the universal system for defining colors that is set by the CIE [46]. In the color analysis experiment performed in this study, the CIELAB color space system was used, and the color difference (\(\Delta\)E) was calculated using the following equation:  
\[\Delta\mathrm{E}\ =\sqrt{\Delta L^{2}+\Delta a^{2}+\Delta b^{2}} \tag{1}\]  
where \(\Delta L\) is the brightness difference, \(\Delta a\) is the difference between red and green, \(\Delta b\) is the difference between yellow and blue, and \(\Delta E\) is the color difference. In the L*a*b color space, L* represents brightness (luminance) and a* and b* represent chromaticity [47; 48]. CIELAB provides a perceptually linear color space or a result of the closest color space [49]. The Euclidean distance between coordinate points can be designated in the CIELAB color space to perceive the color difference of HSCs according to age, and the color difference can be evaluated according to the stage [50]. Here, 16 points were selected on the surface of a \(270\,\mathrm{mm}\times 270\,\mathrm{mm}\) specimen, as shown in Figure 7, and the color difference and chromaticity were measured at these points at each day of age. In this study, the color difference of each specimen at 3 days of age was set as the standard value, and the standard value and color difference (\(\Delta\)E) were analyzed by age using \(\Delta\)E as the sum of vectors of each element of the L*a*b color system, as expressed in Equation (1). When the color-difference range according to the age of HSCs is calculated using Equation (1), the color-difference range suitable for the characteristics of the product can be designated by considering the matching color range of colored UHPC. The designated color-difference range can serve as a color-development specification in concrete products for the addition of pigments. As presented in Table 3, the color difference of specimen P tended to increase with age, whereas the pigment content in each sample remained constant. However, for other samples, the color difference increased or decreased at 7 and 28 days of age. The color change with age was the smallest in the red-pigmented samples, followed by specimen G10. Specim G5 exhibited the largest difference, and the color difference between 7 and 28 days was 4.5. Figure 6: XRD patterns of samples showing the specimen powders produced at 28 days of age, where peaks B, C, and Q represent belite, calcite, and quartz, respectively. Figure 8 shows the results of analyzing the chromaticity of HSCCs by age, and Table 3 presents the results of analyzing the color of each sample by age. The \(x\)-axis of the subplots of Figure 8 represents the red-green axis, and the \(y\)-axis represents the yellow-blue axis, using a* and b* as color coordinate indices. The a+ direction is red, and the a\(-\) direction is green. Furthermore, the b+ direction is yellow, and the b\(-\) direction is blue [51]. Each pigment was located in a certain section (A to G) regardless of age. However, slight differences were observed depending on the age because of the humid environment during the standard curing. Furthermore, in the thermogravimetry results, when the pigment content was 10% for yellow and green in the range of 45-210 \({}^{\circ}\)C, which was the hydrate combustion and dehydration section, the proportion of \(\Delta\)m was lower than that at a pigment content of 5%. This result indicated that a slight difference occurred during the reaction between the pigment and the moisture content. In contrast, for the red pigment, the proportion of \(\Delta\)m was similar regardless of its content, confirming a tendency similar to that observed in the chromaticity analysis.