We filled up an ice tray with water adding one drop of food colouring in each segment and put in freezer overnight.
Once frozen, we put the ice cubes into a clear tub of water and the children watched and played with the melting ice, and watched the colours blend and swirl.
You could use two colours at a time to see what happens when they are blended (e.g. Blue and yellow- turns the water green). Ask the children to explain what is happening to the ice, ask them to use words to describe what it feels like to touch, and notice whether the ice is sinking or floating in the water.
A variation of this activity is to freeze children's acrylic paint with a bit of water in ice-trays and children will enjoy drawing and painting with the melting cubes on white paper.