User blog comment:CNBA3/Gajeel's Iron Dragon Magic/@comment-5894890-20130730042813

What FTGM said earlier. Don't over think it :P. Also it's not just dragon slayers, notice how Lucy lets out an yellow aura when performing Urano Metria. I think the aura itself just helps portray the character better. Ex. Hades= black aura, indicating his 'evil' character- not always the case though.