Here's your answer: The card number.If it's Numbered #9 - 16, it's a gold card, #17-24 is bronze.Same with Ability Cards. #33-40 is Blue, whereas #41-48 is GreenMost of the limited edition cards are blue, with the Special Attacks and Traps (Besides Legionoid) being red.
A lot of the SAs and traps are Blue and Green now, but they were mostly all Red before (back in B2)And the numbers thing was mentioned by DB already lol (Hopefully being told it 3 times gets him to use it)