## 3 Results and Discussion
### Colorimetric Analysis  
Color change tests on samples using a colorimeter based on the CIELab system were conducted. The \(L^{*}\), \(a^{*}\), and \(b^{*}\) represent each of the three values the CIELab color space uses to measure objective colors and calculate color differences [21]. The \(a\) and \(b\) axes are placed at right angles to each other and indicate the color tone. Parameter \(a^{*}\) ranges from green (negative values) to red (positive values). In contrast, negative \(b^{*}\) represents blue, and positive \(b^{*}\) corresponds with yellow. The \(L\) parameter denotes luminance (brightness) and describes the color in the range from black to white. A negative value indicates a change to a darker shade, while a positive value means a change to a lighter shade. For black, its value is 0, while, for white, it is equal to 100. Using the CIELab system, differences between the color shades are determined. It is the distance between two points in a three-dimensional space, which can be written as [22]:  
\[\Delta E=\sqrt{\Delta L^{2}+\Delta a^{2}+\Delta b^{2}} \tag{1}\]  
\[\Delta L=L_{2}+L_{1} \tag{2}\]  
\[\Delta a=a_{2}+a_{1} \tag{3}\]  
\[\Delta b=b_{2}+b_{1} \tag{4}\]  
The assessment of the color change is made on the basis of the change of the \(\Delta E\) parameter, the ranges of which are listed in Table 8. \begin{table}
\begin{tabular}{c c} \hline \hline
**Marking of the Sample** & **Full Sample Name** \\ \hline Ref. 6 & PLA WOOD reference \\
6.4 & PLA WOOD + silver nitrate dissolved in water and isopropanol mixed in a 1:1 ratio \\  & (saturated solution) \\  & PLA WOOD + iron (III) chloride dissolved in water, isopropanol, and glycerine mixed in a \\  & 1:1:1 ratio (saturated solution) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Samples presented in Figure 11 and their markings. Figure 11: L-FDM-printed cylinders of PLA WOOD (**A–C**) modified with: (**A**) reference sample not modified marked ref. 6, (**B**) silver nitrate marked 6.4, and (**C**) iron (III) chloride marked 6.5. \begin{table}
\begin{tabular}{c c} \hline \hline
**\(\Delta E\) Value** & **Meaning** \\ \hline \(0<\Delta E<1\) & A normally invisible difference \\ \(1<\Delta E<2\) & Very small difference, only obvious to a trained eye \\ \(2<\Delta E<3.5\) & Medium difference, also obvious to an untrained eye \\ \(3.5<\Delta E<5\) & An obvious difference \\ \(\Delta E>5\) & A very obvious difference \\ \hline \hline \end{tabular}
\end{table} 
Table 8: \(\Delta E\) standards. Figure 12a shows the color changes (\(\Delta E\)) of samples printed with the L-FDM method, in which dye solutions were used as modifiers, i.e., alkaline blue, methylene blue, methyl orange, eosin, fluorescein, transparent red dye by Moldoo, and black printer ink by Canon. The values on the Z-axis corresponded to the markings of the tested samples. The filaments were passed through a reservoir containing solutions of said dyes in methanol and printed in the form of trabeculae. The names and compositions of individual compositions were collected and presented earlier in Table 2. Figure 12b shows the color changes for cylindrical samples modified with commercially available dyes and solutions of rhodamine B in a mixture of water and organic solvents (water:isopropanol and water:isopropanol:glycerine) (see Tables 3 and 4 for a detailed explanation of the determinations). Figure 12c shows the \(\Delta E\) values for composites containing fibers and wood dust with both color modifiers (5.1-5.3) and metal salts (6.1-6.3, sample designations Tables 5 and 7). Figure 12d shows color changes for the composites based on PLA and PET-G modified with metal salts (silver nitrate and iron III chloride) (Table 6). Figure 12: Quantification of color changes for printed samples.