Hermaphroditism in fish has been well documented. Protogynous hermaphrodites (female to male sex change) are normally found in a polygamous social hierarchy. The male is the dominant within the group, followed by the largest female and it is common to observe aggressive behaviour as an exercise of dominance. Most of the studies relating to sex change used tropical species as models, however, little is known about temperate species. In this research, Notolabrus celidotus, a temperate protogynous wrasse, was used to investigate sex change. The overall goal was to develop an accurate methodology to assist with colour analysis of an external landmark. Ten males and ten females had pictures taken from the whole body. Digital photography and computational image analysis programs were used to help quantify the colour. Results were found through circular statistics, as hue is read as an angle. It was found that the anal fin presented the most significant mean variation of hue, within the body (female = 95.353 ± 3.16°; male = 141.138 ± 17.23°). The technique was then used to quantify the colour changes of the anal fin as individuals transitioned from female to male. Histological examination was also analysed and classified according to the transitional phase of every fish. The developed technique was able to track the colour changes and allocate them into distinct sexual transition stages. Statistical analysis showed that major colour changes began at the mid-transitional stage and progressed until the terminal phase male was achieved. The histological analysis of the gonads contributed to the study demonstrating internal changes through each sexual stage, additionally showing connection between the changes internally and externally. Through the development of a method to quantify external colour in relation to sexual state, this work creates opportunities for future studies. Investigations of sex change in this species can now be conducted without the need for invasive or destructive sampling to identify sex changing individuals. Further to this, individuals that are at different stages of sexual transition may be identified and selected for sampling by their external colour. Overall, this study was able to quantify and allocate external colour changes into distinct stages that map the restructure of the gonads during sex change.