Arrange the skittles on the plate to form a circle around the edge, fill the space in the middle with water and wait…
Skittles are made mostly of sugar and food colouring. These two items are particularly susceptible to dissolving when exposed to water or any other liquid. This provides you with the initial colouring of the water.
The colour then spreads out over time, this is through diffusion. Diffusion is what happens when an area of highly concentrated molecules or items moves to an area of low concentration. This doesn’t just give you some pretty cool science but gives you a piece of art!