Do hard hats have a validity date and what's the best way to manage this?

They have a manufacture date stamped on them, and I understand that general guidance is replace, when damaged, subjected to a 'hard' knock or otherwise on a calendar basis of 3 years. One way of managing this might be to have all hard hats of one particular colour and then change them all for another particular colour at the end of the calendar expiry period that you chose. This may mean that you have to replace some during that period because of damage and knocks and therefore replace some at the change of colour earlier than you might but depending on your circumstances that shouldn't hopefully incur a significant cost. It might not be quite as easy as that if you use a lot of contractors on site. Just my evening ramble. I'm sure the construction chaps will have some ideas.