Redshirts have some aberrant behavior that can be advantageous. It should cost 4 diamonds to make it content or 6 to make it happy, but a deduction from the city screen suggests that it only costs 2 diamonds to turn a redshirt into a happy. The city produces 15 diamonds: evidently 12 are used to turn 6 redshirts happy, and the last 2 are used to turn 1 blackshirt into a content. On the other hand redshirts still count double when the temple, cathedral, colosseum, and Bach are used to pacify them.
On the other hand, city can spend only city_size*2 luxuries max (any additional lux's totally useless), so this behavior is not too bad. If it costs 6 diamonds to make them happy it would be impossible to start WLTPDay if you have too many cities. So I don't even know if this is a bug or intended behavior. And dunno should I reproduce it or not.