Skittle colours and sugar gradients

I've seen a lot of versions of this experiment online, but most descriptions seem to stop at admiring the pretty coloured effect.  We had a go, and I tried to explain a bit of the science behind it to my son, whilst stopping short of fully explaining that Skittle sweets are edible...

I picked up some Skittles in a discount shop, but you could use any multicoloured sugar-coated sweets with a hard shell (e.g. M&Ms).  You arrange them on a plate to form the outline of a shape, and pour water inside your shape so that it reaches the sweets.  The water dissolves the sugar and the colours start to diffuse along a concentration gradient, away from the sweet.

We tried making a circle, and the small boy helped to arrange the Skittles on a plate. We also set up a second plate with a spoonful of sugar in the centre of the circle.  He was a little more generous with the sugar than I'd intended, so it's a heaped teaspoonful! The idea behind this was to provide a high concentration of sugar once the water was added, to see if this reduced the diffusion of colour from the coating of the sweets.

Skittles on plates with sugar (L) and without (R)

We poured in enough water to reach the sweets and sat and watched. The colours quickly started to spread out from the sweets towards the centre of the circles. The colours didn't mix with each other, but formed distinct stripes. The colours of the sweets on the plate with sugar moved more slowly towards the centre of the circle.  I forgot to take a photo early on in the process, but the one below shows it after a couple of minutes.

Skittle colours have almost reached the centre of the plate without sugar (R)

After a while, the colours on both plates started to mix (as the concentration of sugar becomes the same across the circle), but the sugar in the centre of one circle kept it free of colour.

No colour in the centre of the sugar-filled circle, even after a further 5 minutes

This is a nice visual experiment but if I'm honest most of the science is lost on a 3 year old. He liked the colours, and I think learnt that they dissolve in water and spread out away from where they start. We will come back to this when he's older, although next time round he may be wise enough to realise that if they contain sugar they are probably edible, and I will have to share the leftovers!