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A simple, non-destructive and objective method is developed and evaluated to quantify bleaching or colour change in coral in terms of its percent whiteness. Digital photos of coral colony were taken with a photoquadrat marked with black and white bands around a metal frame to serve as colour references. Photos taken were then converted to a black and white grey-scale and intensities of the coral colour were determined with reference to this grey-scale using computer image analytical technique. A completely bleached coral gave an estimated percent whiteness that is close to 100%, an estimate that agrees well with the visual image of a bleached coral. The percent whiteness estimates were found not to be significantly affected by different levels of irradiance and turbidites under simulated conditions. Colour change in seven common Hong Kong coral species were further evaluated in situ and shown to exhibit an increasing trend of percent whiteness from January (winter) to April 2012 (spring), reflecting an increasing degree of bleaching. Using Goniopora lobata as an experimental species, estimated percent whiteness was shown to be negatively correlated with its zooxanthellae density and chlorophyll a concentration. This suggests that this method could allow repeated monitoring of individual coral colony over time to detect subtle variations in its colour as a general assessment of its health. Furthermore, reference percent whiteness of different coral species can be standardized for comparison of colour change over time to identify the critical level of percent whiteness that is associated with bleaching. Thus, this method should prove to be an easy and objective tool for rapid field assessment of coral health.