Prefer black. Prefer plastic, so card can work without being removed. Plastic ones can be found on Amazon: Oyster card holder (The Great Wave art or cat one is nice) Linked one below is a leather black option (same size as current one) that is a good example of second option but I'm unsure on the quality so the link is just an idea.
There is more to look out, may be more added so have a look at the website: Ngoc Novelty. Link is to a specific one I liked. I also like the Unlock necklace and 8 bracelet from the stay collection.
Website