I'm sorry, but they're colored-contacts.
Asian girl in high school used to lie and say she had naturally bluish eyes despite the fact that she was Asian, and we found out she was lying when one of them fell out and she had to take them off for the rest of the day
Im very aware that most JAV models are using contact lenses but MIO YUMOTOs eye color is natural. I double checked all of her movies specially her latest uncensored movie in Tokyo-Hot. She definitely has a very light brown sort of very weak green eyes.
You can easily tell it is natural eyes because the light passes through her cornea and the color is reflected on the FLAT surface of her iris (colored part ). If those were contact lenses, the reflection of the color of the eyes would be on the curved surface of her eyes (cornea).
Also if you are going to watch her movies, her pupil changes in size depending on the light conditions of the scene. If those were contact lenses, the diameter of her pupil will be fixed and it will not enlarge if the light is dim.
It is quite easy to tell if someone is wearing contact lenses.
EYE COLOR inside the eyes on FLAT SURFACE = Natural
EYE COLOR on outside the eyes on CURVED SURFACE = Contact Lens