What are the orange stripes and fire that Eren's Titan body gets when fighting the female Titan for the second time? In the second fight with the female Titan, at the end, Eren's Titan gets some orange lines and fire. What is that? Is it some special power particular to his Titan just like the female Titan can harden part of her body when fighting?
I believe it's heat from the extreme regeneration he's undergoing. Normal Titans emit steam from wounds as they heal; it makes sense that accelerated healing might emit more heat to the point of glowing. (From a chemistry/biology standpoint this could be seen as a waste of chemical energy, but we're talking about a person who can summon a lightning bolt by biting his hand and somehow use it to grow a fifteen-meter body around himself!)