Son number 3 is at the age (21m) now when he can spot things are different colours. For this activity I found the large buttons in my button jar, and gave him 3 containers, and assisted him a few times in sorting the buttons into their correct colour pots. After a few goes he was sorting them himself and giving himself a clap too :)
He litrally spent an hour playing with a few buttons and pots. when he had a banana event the skin went into the correct colour pot :)
When doing an activity like this, make sure the buttons are large enough that they wont be swallowed.