Introduction:
The addition of nanoparticles to poly(methyl methacrylate) (PMMA) changes the colour and optical properties of the new biomaterial, which might result in limited or impossible application of the aforementioned modification in production of denture bases.

Aim of the study:
To determine the manner in which incorporation of zinc oxide changes the colour properties of PMMA and of the permanence of the obtained colour after subjecting the material to colouring agents.

Material and methods:
The test group was composed of 2.5%, 5%, and 7.5% of PMMA–ZnO nanocomposites, the control group was Superacryl Plus, a thermally polymerised acrylic material (Spofa Dental, Jicin, Czech Republic). The colouring agents used in the test were coffee (CO) and red wine (RW), in the control group it was distilled water (DW). The samples were subjected to a colorimetric test with the use of a digital colorimeter and recorded within the Red/ Green/Blue (RGB) colour space.

Results:
The change of colour for PMMA and individual nanocomposites increased in line with the content of nanoparticles. In the colour permanence test, considerably higher ΔE values were observed for samples coloured with red wine. In comparison to pure PMMA, smaller concentrations of nanocomposites behaved similarly or were discoloured to a slightly greater extent. The 7.5% nanocomposite behaved differently: they proved to be much less stable in terms of colour compared to other nanocomposites.

Conclusions:
1. The modification of PMMA with ZnO nanoparticles is acceptable in aesthetic terms. However, the slight whitening of the material must be discussed with the patient before its potential clinical application. 2. Due to the lower colour stability of the material, the use of the 7.5% nanocomposite needs to be confronted with the possible benefits arising from its positive microbiological action.