Board Thread:Questions and Answers/@comment-32768599-20170831094647/@comment-294304-20170831130334

Hair color change seems to be just a sign of magicraft overuse and isn't specifically tied to EMIYA creating his own magic circuit (and seeing as Shirou learns not to do that in just about every route of FSN, assuming that any incarnation of EMIYA does is silly.)

Several other magi, such as the Aozakis right off the top of my head but even Sakura having purple hair, have altered physical features due to exposure and overuse of magicraft, but yeah EMIYA is the only one who has abused his magicraft so much that it darkerned his skin. Going by what we see of a younger Nameless (EMIYA's EXTRA-verse counterpart), his hair turned white first then his skin darkened btw.