User blog comment:B j/The Five Sabertooth Mages/@comment-4449950-20111217104141

Don't know if this has anything to do with anything, but in the Chinese elements, each element has a colour. The Chinese elements are Water, Earth, Fire, Metal and Wood.

The colour for the element Metal is white, which could relate to Sting the 'White Dragon Slayer'. This also relates to him eating the metal arrowhead - if that's what he was eating.

The colour for the element Water is black, which could relate to Rogue the 'Shadow Dragon Slayer'.

So, if I had to put forward an idea, I'd have Sting as some form of Metal Dragon Slayer and Rogue as the Water Dragon Slayer - he would also be able to give Natsu a good fight (fire vs. water).