The square turns a lighter gray when you're purchased it and the 'passive' elements are active like the exp/gp/drachma gains. It lights up the color that matches the card when the special event at the bottom of the card happens. Ie. If you bought Poseidon's card, it'll shine up light blue briefly when a bubble appears. Or if you've gotten the Aphrodite card it will light up pink briefly when a rare urn appears.
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The square turns a lighter gray when you're purchased it and the 'passive' elements are active like the exp/gp/drachma gains. It lights up the color that matches the card when the special event at the bottom of the card happens. Ie. If you bought Poseidon's card, it'll shine up light blue briefly when a bubble appears. Or if you've gotten the Aphrodite card it will light up pink briefly when a rare urn appears.