The Smart Lid system was developed to save your precious
face from being burned by hot coffee (and retailers from lawsuits) by turning red when it's "too hot" to drink. It also tells you whether the lid is properly sealed by showing a broken rim line where the lid isn't attached.
Personally, I think this is waste of money, since my real concern is if my coffee's hot enough. If my tongue isn't tingling afterwards, no. Really, coffee's only too hot if it makes you blister, and that's only happened to me twice, maybe seven times tops.