I've confidently picked 8+blue and is now trying to understand why I personally did that. I think that maybe the text of the puzzle is not quite unambiguous. The question states "test a card" followed by "which cards", so this is what my brain immediately starts to check - every card one by one. Do I need to test "3"? No, not even. Do I need to test "8"? yes. Do I need to test "blue"? Yes, because I need to test "a card" to fit the criteria. And lastly "red" card also immediately fails verification of a "a card" fitting that criteria.

I think a corrected question should clarify in any obvious way that we are verifying not "a card" but "a rule" applicable to all cards. So a needs to be replaced with all or any, and mention of rule or pattern needs to be added.