I think it would be awesome to let the customer choose a colour for the debit card. the reason for that is to make it easy to pick the right card when needed. my wife, for example, has her own Monzo and another card for the joint account, and quite often she picks the wrong one.
Also, it will be great if you let the customer put their mobile/email number on the card. if the card lost, it might be easier to return it to the owner rather than the bank.