A study was conducted to determine the effects of increasing the levels of dietary carotenoid-rich microalgae biomass on the skin colouration of angelfish (Pterophyllum scalare). A natural microalgae product characterised by a high content (approximately 5% w/w) of the carotenoid astaxanthin was included in a basal diet at 0.25, 0.50, 1.00 and 2.00 g per 100 g as a substitute for wheat flour. Each diet was fed at 3% of live weight to three replicates of 10 fish each for a period of 28 days. Final weights of replicates were determined as indications of growth. Further, non-invasive techniques were applied to evaluate skin colour. Whole body images of individual fish were processed using specialised computer software to measure the different components of the RGB, HSV and CIE L*a*b* colour evaluation systems, which were also assessed by three independent judges familiar with the rearing and trade of ornamental fishes. No significant influence of the microalgae addition was detected (P > 0.05) on the final live weight of the fish. The colour components Red, Hue, Value and L* were not linearly related (P < 0.05) to dietary microalgae inclusion levels. Where related (Green, Blue, Saturation, a*, b*), correlation coefficients were 0.66 at most. Second-order polynomial regression models showed that maximum or minimum values for colour measurements were frequently outside the range (0.00–2.00 g per 100 g) of the microalgae inclusion levels applied. When visual evaluation (the most common assessment approach in practice) was used, the judges clearly confirmed that the colour changes were directly related to increasing levels of microalgae inclusion.