This is a cool science experiment that you can use in a class room or Sunday school lesson
A rainbow is a symbol of hope. A symbol that God will keep his promises. So any time you are going to talk about hope, God's promises and God's faithfulness you can create this rainbow.
You can also use this experiment to tell the story of Noah and the rainbow.
But personally we also use this to talk about unity - we are all different, but still one!
And what do you think about the fruit of the Spirit? So many expressions of the same Spirit in different colours...