Go outside on a perfect Scottish summer's day (I know, they are rare).
Ask them to find natural items of every colour:
Spread out some paper and poster paint:
Read a story about colours (The Blue Coat from The Story Tree).
Then, go to the pond to wash off the paint. Or to go fishing. Or to splash and squelch the mud/catch tadpole/play pirates etc.
Ingredients for a perfect day.