It surprises me a bit that Wendy's hair color change from blue to pink when her dragon force is activated. Neither Natsu's nor Sting's hair color changed when their DF is activated, but Wendy did.
Since in the next arc we would face the opponents that are much stronger than before, I guess Gajeel and/or Laxus might have to go DF as well. We haven't seen their DF form yet, so we can't tell if their hair color might change like Wendy's, but I surely hope so.
So what hair color do you want to see on Laxus and Gajeel? Personally, I would like to see a blue/purple-haired Laxus and a silver-haired Gajeel. What do you think?