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An automated system for measuring plant leaf colour, as an indicator of plant health status, has been developed for plantlets growing in a modified micropropagation system. Using a custom built robotic arm, sensors located on a pan and tilt system at the end of the arm monitor plant growth and the ambient growing environment. Sensors include a compact colour zoom camera, RGB (red, green and blue) colour sensors, and environmental sensors. Leaf colour sensors provide information, in a non-destructive manner, on the health status of tissue by comparing the sensor outputs to pre-determined optimum values. These low cost colour sensors can be incorporated into a continuous automated system for monitoring leaf colour of growing plants. Subtle colour changes can be an early indication of stress from less than optimum nutrient concentrations. When combined with automated image sensing for growth analysis, and environmental sensing (RH, CO2 and temperature) in a controlled environment, optimised rapid growth with minimal human input can be achieved using a modified micropropagation system. In this paper we detail the calibration technique for a RGB sensor and compare it with a high end spectrophotometer.