Each Skittle has a thin sugar-and-dye coating on the outside. When water contacts it, the coating dissolves — and that sugary solution is slightly heavier (denser) than the plain water around it. Denser liquids sink. So the coloured sugar solution flows along the bottom of the plate toward the centre, where the water is still clear and dilute. The dye travels with the sugar, creating a coloured stream moving inward from each Skittle toward the middle of the plate.

When two streams from adjacent Skittles meet, they stop — and the boundary line holds. This happens because both streams dissolved from identical candy in the same depth of water at the same time. They have virtually the same sugar concentration and the same density. Since neither stream is heavier than the other, neither sinks beneath its neighbour. And since there is no concentration difference between them, there is no gradient to drive one stream into the other. The streams meet as equals, and equals have no reason to mix. The boundary holds.

When you add a sugar cube to the centre, it dissolves rapidly into a much more concentrated solution than the Skittles produced. Now the centre has the highest sugar concentration on the plate — denser than the surrounding coloured streams. This reverses the gradient: instead of the centre being the low-concentration destination, it has become the high-concentration source. The dense central solution flows outward along the plate floor, pushing the coloured boundaries back toward the edges. The rainbow retreats.

This is also why warm water produces a faster result: heat gives particles more energy, so they move faster and the coating dissolves more quickly. The streams form sooner, but the boundary effect is the same — because the concentration of each stream is still equal to its neighbours, regardless of how fast they arrived.