According to a number of studies, 3D printed dental resins have better optical stability than PMMA (polymethyl methacrylate) resins for removable prosthodontics [22; 23; 24; 25]. The optical quality of the denture base resins is a property of paramount importance because it ensures the aesthetical appearance and patients' comfort [26]. In comparison with the conventional heat-polymerized PMMA for removable dentures, 3D printed dental resins have a number of advantages. They include simplified laboratory protocol, time effectiveness and more efficient planning of the prosthetic restoration [27; 28; 29]. The shortcomings of both types of dental materials include discolourationover time, which affects negative the aesthetics and the long-term success of the prosthetic treatment [30; 31]. Colour determination is possible via different visual and instrumental methods. The visual method consists of a clinical evaluation performed by the dental practitioner using different colour shade guides [32]. In comparison, the instrumental techniques are more accurate and exclude the possibility of subjective errors. They include colourimeters and spectrophotometers, which detect the colour values in accordance with the CIE L*a*b* colouring system. The CIE L*a*b* system defines the colour space with both chromatic value and saturation of the L*a*b* coordinates: L* measures colour bleaching (a value of 100 corresponds to perfect white and 0 to black); a* measures colour in the red (a* > 0) and green (a* < 0) dimensions; and b* measures colour in the yellow (b* > 0) and blue (b* < 0) dimensions [33]. Delta E is defined as the difference between two colours in an L*a*b* colour space, which is referred to as a mathematical formula. This information is significant for the evaluation of the distinct values. The CIE L*a*b* formula is based on the Euclidian distance, which represents the distance between two points in a three-dimensional colour space [34]. In addition, the vision is more susceptible to particular areas of colour and less susceptible to others. This is a fact that the formula does not take into consideration. The deficiencies of the eyes were compensated using further colour equations [35]. Colours in a highly saturated area are determined less significant during the evaluation than colours along the gray axis. This is the area where the vision is most sensitive. In the case that the delta E value of 1 is visible to the untrained eye, this does not apply for the highly saturated region but it is true for the gray axis [36]. Spectrophotometry is a type of method that is explained by the theory of electromagnetic spectroscopy. This process is applied with the quantitative measurement of the reflection of a material as a function of the wavelength [37]. The spectrophotometers are used for the quantitative evaluation of molecules. They are based on the amount of light that is absorbed by a variety of compounds. Spectrophotometers measure the intensity of a light beam at different wavelengths and have the ability to obtain wide swaths of the electromagnetic spectrum [38]. Significant features of spectrophotometers include the spectral bandwidth. This is the scale of colours that can be transferred through the test sample, the percentage of sample transmission, the logarithmic span of sample absorption, and sometimes a percentage of reflectance assessment. Spectrophotometers can also be designed to estimate the diffusivity on any of the listed light spans that usually enfold around 200-2500 nm applying different adjustments. Within this extent of light, calibrations are necessary using standards that vary in type depending on the wavelength of the photometric resolution [39]. The aim of this in vitro study was to investigate and compare the colour changes occurring in these two types of denture materials in various staining agents for the selected immersion periods.