A nice core game mechanic! I'm not sure the grey cards are necessary, though - if I understand them correctly, they would rather add confusion than add an alternative mechanic - i.e. is 2+2 different from just 4?
I would also recommend making it up to 3 wins, not 5-6 rounds - it becomes tedious when you have already won 4, but it keeps going.
Also very cute art and great sound work!