Board Thread:Questions and Answers/@comment-32768599-20170831094647/@comment-78.194.252.210-20170831115051

When he use magic, Emiya create his own magical circuit before tracing. It is very dangerous because it can destroy the body and even kill himself if something miss. He do it all the time, which exposed his body internally to a great amount of mana. This mana destroyed his hair pigmentation, making them white, and burn his cells, tanning him. Du to this, his alter version is even more darker, because he used it even more usually.

At a certaine point, it must be abusing this that tanned him. Not just doig it sometimes.