Glue (optional unless you are making the rainbow permanent)
Pull out your Rubbermaid totes of all your children's Lego pieces and get them building some rainbows.
Let them use their imagination creating the clouds or the sky and then the building the rainbow with the colours of their Lego pieces.
In the Lego movie, the characters were afraid of the "Kragle" which of course was crazy glue. If your children don't mind giving up some of their pieces, they can create 3-dimensional art on a canvas and you can glue the pieces down.
Creativity is limitless. Dump out the pieces and let your children create.