L* , a* , and b* parameters were measured and compared for both the on and off-line procedures to correlate the reflectance spectra to the colour modification (Table 1). The on-line analysis revealed that a* value, related to redness, was reduced during the treatment, while L* and b* increase from 5 min to 60 min, proving an increment in brilliance and yellowness of the sample. The off-line measurements of colour parameters showed a similar behaviour. The overall changes in colour ΔE were also calculated (Table 1). The increase of on-line ΔE values during the process from 0.8 ± 0.4 at 5 min to 4.6 ± 1.4 at 60 min, highlighted a quite distinguishable