How to measure sugar colour?

The fast and easy way is to make a visual evaluation. Unfortunately, the perceptibility of the colorants depends heavily on the shape and size of the crystals. Large crystals look less bright than small crystals. Colorants on the crystals' surfaces are more visible than included colour. Dry sugar looks brighter than moist sugar. These facts have led ICUMSA (International Commission for Uniform Methods of Sugar Analysis) to introduce two kinds of colour measurement for sugar: One for visual colour of the sugar crystals (colour type) and one for colour of the sugar in solution (solution colour). When the sugar is dissolved, any influence from the crystals and their shape or moisture content is eliminated. The exact procedure for the laboratory measurement of solution colour can be found in the ICUMSA Methods Book (Bartens, Berlin).
Any method for measuring sugar solution colour based on visual evaluation, like video cameras or CIELab based colorimeters will need a recalibration every time any of the disturbing influences mentioned above changes just slightly.