Do you draw severely jaundiced people? A human eyeball is white, outside the iris.
The color is white. The shading is usually not.
Take a photograph, properly color correct it, and see what color the "white" actually is. It will vary a lot, depending on the color of light falling on the eye, and be tinged by the blood vessels in the eye.
What it won't be is #ffffff.
I think he/she took offense to the "darkest part of the face" portion of the comment; which is clearly false for the majority of the earth's population, particularly since no one draws the iris white so the original comment was clearly referencing the "white" portion of the eye.
If you draw eyes using white paint, they will always look wrong until you acknowledge that the actual appearance of an eye is almost always some subtly colored shade of gray.
Most of the visible part of the eye isn't the eyeball, it's the iris. Take a look at some photographs: the iris and pupil fill more than the entire vertical height of the eye, and about half of the horizontal width. Given the shape of the eye, that's a majority of the area.
The eye ball is white but the eye is not especially if the eye is considered to include the eye socket. Unless you are lying on a dentist's table, your eyes are dark when compared to the rest of the face because the eye socket creates shadows. Yes the eyeball is white but the eyes are in fact dark.