The Science Behind It:

When rainbow candies come into contact with water, some of their water-soluble dyes and sugar dissolve in the water to form a solution. As the candies dissolve, the density of the solution around them gradually increases. To achieve balance, these solutions will gradually spread from areas of high density (around the candies) to areas of low density (the centre of the plate), so that all parts of the water with sugar and food colouring have roughly the same density. This is what happens when the rainbow starts to move away from the candies! Scientists call this process diffusion.