Please change the coloring on the recycle button to reflect whether you already used it. It is always gray, though it does change to a nice blue when you drag your card on it.
Maybe green for unused, and gray for used.
It actually changes color - but may not be too much. The color of the arrow is white when available and grey when used. But you're right - changing the entire circle color is a lot more noticeable and it's an easy change.