Ha! That's pretty cool.

My first thought with this card was that it could be "gamified" into something that kids would probably love. A clique-y, social thing where kids could "pour" some of their fluid into another's card, with NFC or something. User preference colors that don't change when "poured" could help indicate how many different people have interacted with your card.

But enough spitballing. There's no killer idea there. Just something I'll be amused to see show up as a value-add for some other kind of toy, or whatever.