Toilet tanks were boring and soulless until Vital Industries decided to do something about it. If you are not content with the safe color choices brought to you by your manufacturer of choice, why not add a toilet sticker? There are a few of them to choose from. They have SCUBA divers, motor scooters, bicycles, puffer fish and cowboys. From what I understand, they are easy to apply and removable (though you will need some cleaning product and a razor blade to do the cleanup properly).
Available from Etsy for $20 each.