Hi Sinéad,
You could adapt that demonstration to become more inquiry focused by showing the rainbow happening, then asking the children what they wonder or would like to try differently (and why they think it would make a difference).
for example: ” I wonder if we could make the rainbow faster… what would happen if we used different paper (like greaseproof paper)? or put the colours in a different order or ….”
These set demonstrations can serve as a wonderful prompt, but the science learning by inquiry is based on the children using their own understanding to develop their ideas further. For that they need to have familiarity and some working mental model of the scenario and be challenged to explain it further and to use it to predict what will happen.