How does the sugarcane colour change from black to white colour?
Raw sugar is originally yellowish or brownish, which is the color of its ingredient, molasses. To refine the sugar (though brown sugar is healthy as well), certain chemicals are added to it.
One common process is the sulphitation process wherein Sulphur Dioxide SO2 is allowed to pass through cane juice till the water content is evaporated. This bleaches the colored impurities of sugar to colorless.
To further refine, we should wash away the outer coating of the raw sugar crystals, in a process called "affination". The remaining sugar is then dissolved to make a syrup, in about 70% w/w of all solids in it. To it is added Phosphoric Acid and Calcium Hydroxide; which precipitates Calcium Phosphate. These particles entrap some impurities and absorb others, and then float to the top of the tank, where they can be skimmed off. Alternatively Calcium Hydroxide and Carbon Dioxide is used to precipitate Calcium Carbonate to absorb the impurities.