Tip a pile of clean socks on the floor and sit beside it with your child.
Start two piles yourself, saying the rule out loud: 'dark ones here, light ones here.' Do three or four so the pattern is visible before you hand over.
Then pass socks one at a time. Name the colour as you hand each one. Let them place it anywhere and say the rule again rather than moving it.
Sorting by one property while ignoring others is the foundation of classification, and doing it on a real chore means the rule has a visible purpose. Ignoring shape and size to attend only to colour is genuine cognitive selection.