God Serena Theory; God Serena's other three elements
So, this has been aching me for a long time now, we've only ever seen five of God Serena's impressive Eight Dragon Slayer Elements (Cavern Dragon, Sea King Dragon, Purgatory Dragon, Gale Dragon, and Lightning Dragon) we know he uses Lightning Magic as we've seen it crackling around his hand when he went to kill Warrod, and judging by Jura's shocked reaction when God Serena began using more elements I think the first round before he applied slightly more effort into the fight he was only using lightning magic to fight them. So we know five of the elements, I believe the color of the dragons implies what elements they are, pink would be fire, orange/brown is earth, green is wind, yellow is lightning and the darker shade of blue is water. So, this leaves us with light blue, whiteish purple, and purple. Here's what I think they are. Note, this is only speculation and not fact.
Light Blue Dragon; I think the light blue dragon stands for the element of ice, since he uses water it isn't a stretch that he'd add ice into his body as well. If this is the case it could be devastating if he added ice with his Cavern Dragon Attacks, making combination attacks with them.
White Dragon; I believe that the white dragon stands for Light Magic, as a powerful element I see God Serena wanting to add it to his arsenal, he would be able to do a lot with light magic.
Purple Dragon; The purple dragon should stand for shadow/darkness magic, it would make sense as it would be extremely powerful and something he can whip out when he needs a quick escape as we see with Rogue and Gajeel they can go into the shadows, I don't see God Serena passing something like that up given the choice.
Anyway, this is just my opinion on what his other elements could be and my explanation of how I came to this result. What do you think about my speculation? Do you agree or do you disagree, if you disagree what do you think the other elements are? Or do you simply believe that the Dragons stand for nothing and are just colorful for... reasons.