Thumbs up :) color detection might not be effective,take for example a kid putting on a shirt with the same color as the card can trigger false detection. QR code can do, if printed somewhere on the card and readily visible. QR codes are somewhat mature.
Try starting out with this.
Open Source QRCode Library[
^]
Also you can use more complex methods such as Dominant Orientation Templates (DOT) as in
http://ar.in.tum.de/pub/hinterstoisser2010dot/hinterstoisser2010dot.pdf[
^]
hope this helps :)