## 3 Results and Discussion
### Colorimetric Analysis
(**a**) Color change between colored bars and the reference sample (PLA); (**b**) color change between colored cylinders made of PLA, PET-G, and ABS compared with the corresponding reference; (**c**) color changes between colored cylinders and samples with silver and iron ions made of PLA WOOD and WOODFILL dyed with rhodamine B compared with the corresponding reference; and (**d**) color changes between samples with silver and iron ions made of PLA and PET-G. Among the tested PLA composites (Figure 12a), the least intense coloration was observed for samples modified with a methanol solution of eosin and fluorescein (samples marked 1.4 and 1.5), while the most intense coloration and the highest value of delta E showed samples marked 1.6 and 1.7. Differences in the degree of coloring resulted from the compatibility of a given dye with the polymer matrix, its solubility in the solvent, and the molar absorption coefficient, which is characteristic of a given dye structure. In the case of the cylindrical samples (Figure 12b), the analysis of the color changes showed that, for the PLA polymer, the highest changes were characterized by 2.2, 2.7, and 2.10, which were water:isopropanol solutions in a 1:1 ratio. The analysis of the collected data indicated that the factor determining the degree of saturation of the material with the dye solution was the solvent. The collected data for the rhodamine solutions indicated \(\Delta E\) = 5.83 for sample 2.6, in which the solvent was water, \(\Delta E\) = 63.04 for sample 2.7 in a H\({}_{2}\)O:iPr 1:1 solution, and \(\Delta E\) = 24.71 in H\({}_{2}\)O:iPr:Gly 1:1:1 (sample 2.8); the obtained results indicated that the water:alcohol solution penetrated the pores of the material, enabling its better penetration and more intense coloring, and the addition of glycerin increased the density and viscosity of the solution. Therefore, its ability to penetrate the composite structure decreased, the wettability of the polymer with water was limited compared to other solutions, and the smallest color change was observed between the reference and the modified sample. Analogous tests were carried out for rhodamine saturated solutions in water, H\({}_{2}\)O:iPr 1:1, and H\({}_{2}\)O:iPr:Gly 1:1:1 (samples 2.9, 2.10, and 2.11, respectively). The smallest color change \(\Delta E\) < 3.5 was observed for samples 2.3 and 2.4 (commercial dyes). Colorimetric tests carried out for composites based on PET-G modified with various rhodamine solutions showed an analogous effect, as for PLA. The \(\Delta\)E for PET-G dyed with rhodamine B solution in H\({}_{2}\)O:iPr 1:1 was 124.97 (No. 3.1), and the \(\Delta\)E of rhodamine in H\({}_{2}\)O:iPr:Gly was 62.66 (No. 3.5). PET-G was characterized by a higher saturation of the modifier compared to PLA. Dyes such as navy blue fabric dye (sample 3.7) and transparent yellow dye (sample 3.6) were the least effectively introduced into the polymer matrix (\(\Delta\)E 3.82 and 0.92, respectively). The ABS polymer was characterized by an intense color change after the printing process, with green dye and rhodamine B solutions compared to transparent PLA and PET-G (samples 4.1 and 4.2). The \(\Delta\)E for the samples of PLA WOODFILL and WOOD composites samples (Figure 12c) reached high values (above 11). These composites had a porous structure due to the presence of a filler in the form of fibers and wood dust. The result was a better ability to absorb solutions with liquid dyes and modifiers into the material, thanks to which, the L-FDM prints were characterized by an increased content of dye in the polymer matrix and, thus, more intense coloring. Modification of the polymers was also carried out in the presence of solutions containing metal salts, which could be used to introduce specific elements into the polymer matrix and, thanks to the molecular structure, significantly simplify the method of confirming the ongoing modification using microscopic methods and, additionally, intensively stain the material.