I've tried your game and it is indeed a nice implementation!
If will try to give you two small suggestions to make it imho nicer:
- Use also a small icon in addition to card colors (it will make memorization process easier / more fun)
- Implement a timer that will auto-cover the cards if the match failed.
I've found a nice / easy / well done timer API here:
Thanks for sharing your game and happy hacking :)