User blog comment:Artoria's Bike/EMIYA Alter Theory/@comment-29543192-20170113123003/@comment-29363531-20170113141650

That's the same as asking "Why Gil's eyes are red?" and "Why Ishtar's eyes changed color from red to yellow sometimes?"

The answers is:

1. MONGREL!!!

2. Maybe it's related to the power terms? Red is lower version of gold  or have lower power than gold?

In many games or anime, it's a common sense when mc's eyes are glowing or changing color shit's going down