User blog comment:Artoria's Bike/EMIYA Alter Theory/@comment-29543192-20170113123003/@comment-29363531-20170113132100

For me Alter =  An alternative heroic spirit whose origin heroic spirit is already cool but the dark version will be a lot cooler/cuter thus Nasu made it happens. And as Valeth said eyes color just related to mystic eyes, "Powerful eyes shine Gold, while those with powers that reaches the realm of gods have a multi-colored glow, such as the case of Jewel and Rainbow."