This study determined the colour accuracy of a photo-activated prosthetic composite with various thicknesses and shades for the purpose of evaluating the relationship between material thickness variation and colour reproducibility. Four light shades (A1, B1, C1 and D2) and four dark shades (A4, B4, C4 and D4) of a representative photo-activated prosthetic composite (Artglass) for body paste were assessed. The specimens were prepared with thicknesses of 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mm. The L*a*b* colour parameter of each shade was measured using a dental colorimeter (ShadeEye) connected to a computer. The overall colour accuracy of groups of five specimens was compared by means of analysis of variance (ANOVA) and Scheffe's S multiple range test (P=0;05). In addition, L*a*b* colour difference (Delta E*) values between the 3.0 mm thickness and the other thicknesses were calculated. Three-factor ANOVA revealed that all colour parameters were influenced by the thickness as well as by the shade letter and shade tab (P < 0.05). The L* value consistently decreased for all shades as thickness increased, although there was no obvious correlation between the thickness of the material and either the a* or b* values. Among the Delta E* values, the values between the 2.5 and 3.0 mm thicknesses were the lowest, and values were found to be lower than 2.0 for all shades. The colour accuracy of the photo-activated prosthetic composite was related to the thickness of the material regardless of the shade. For acceptable colour reproducibility, a composite material thickness of at least 2.5 mm was found to be required.