They're pretty hard to tell apart as far as I can tell. I wrote a summary of all of the registered cultivars here:
http://www.flytrapcare.com/venus-fly...iety-list.html
The 'Holland Red', 'Royal Red', 'Green Dragon' and 'Akai Ryu' are all very similar. Of the red registered cultivars, I grow Akai Ryu, Green Dragon and Red Piranha. I can't really tell the difference between Akai Ryu and Green Dragon.