Note that the yellow is not in the finished helmet, though the scribed lines are still present.
Later they updated with more Kylo Ren concept art from Jason at Making Star Wars:
This is literally what’s “under the hood”.
This character’s moment in the teaser for The Force Awakens was one of the most memorable, and not just because of the broadsword-style lightsaber. The body-language spoke volumes, as did the tattered cloak, all set against a snowy forest backdrop. It was too cool for mere words to do it justice, and now we get a peek at the mask underneath.