What are the orange stripes and fire that Eren's Titan body gets when fighting the female Titan for the second time?
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!)