## 3 Results and Discussion
### Optical Observations of the Laser-Structured Surfaces  
One of the objectives of the study was to study colours formed when laser processing parameters are varied. The laser fluence used (0.2-1.5 J/cm\({}^{2}\)) was intended to create LIPSSs, also known as ripples. Figure 6 displays the optical observations obtained from the CP Ti's laser-ablated surfaces under the same viewing conditions. It can be observed that the various colours produced on the surfaces of the laser-structured CP Ti depend on the laser processing parameters tabulated in Table 1.  
It can be noted from Figure 6 that surfaces processed by lower laser fluences have more saturated colours than those processed by a higher laser fluence. This can be seen in Figure 6, where the A1 region has a taupe grey colour while the B1 region has a steel grey colour. It has been discussed that the colours formed on the surfaces of the metallic substrate can be attributed to the LIPSS, oxides, or optical active nanostructures formed after laser processing [22]. Through examination of the SEM images in Figures 2 and 3, it can be noted that the surface structures formed varied depending on the laser processing parameters. From Figure 2, region A1 is composed of well-formed LIPSS with a periodicity of 462 nm, while regions B1, C1, D1, and E1 are composed of NC-LIPSS with a periodicity of 722 nm, 727 nm, 736 nm, and 764 nm, respectively. Therefore, the variation of the morphologies of the LIPSS and NC-LIPSS brings the different grey colours observed in the first row in Figure 6. It can be observed from row one in Figure 6, that the intensity of the grey colours increases as the laser fluence increases. This could be explained by the changes  
Figure 6: Optical images of the different regions (A1–E4) of the CP Ti after ps laser processing in scanning mode. The regions were processed using the laser processing parameters in Table 1.  
in the periodicity of the LIPSS, as has been explained by [PERSON] et al. [21]. The greyish colour observed in regions A1, B1, C1, D1, and E1 could be attributed to multiple scatterings of the white light by nanostructures decorating the LIPSS [22]. It can be observed that colours vary in tandem with changes in scanning velocity, hatching distance, and number of scanning passes [44]. This can be seen in Figure 6, where, in region A1, the surface has a taupe grey colour, while in regions A2, A3, and A4, the blue colour changes its saturation level as the levels of the laser parameters change. This can be explained by examining the SEM images in the first row in both Figures 3 and S1. In the first row in Figures 3 and S1, it can be noted that the LIPSSs are formed over the whole surface of region A1, while in regions A2, A3, and A4, the surface is made up of the laser-structured areas covered with LIPSS and unstructured square areas in an alternating manner due to the hatching distances of 25 \(\upmu\)m, 40 \(\upmu\)m, and 50 \(\upmu\)m, used, respectively. As the hatching distance increases, the unprocessed square areas increase, and the intensity level of the bluish colour reduces. This shows that hatching distance plays a critical role in the colourization of the surfaces. The bluish colour observed in regions A1, A2, A3, and A4 could be attributed to the presence of well-ordered LIPSS which acts as optical surface diffraction gratings [21; 22]. From Figure 6, regions C2, D2, and E2 have the same colour. Similarly, the set of regions C3, D3, and E3 and regions C4, D4, and E4 have the same colour. By observing the SEM images of these regions in Figure S1, the morphologies of each set of regions are almost identical. These similar morphologies could be responsible for the similar colours observed. The colours observed on the surfaces of the laser-ablated regions by the ps laser can be a result of inherently generated periodic surface structures on the material which cause optical diffraction effects and are referred to as structural colours [22; 24].