You could solve this by manually adding a vertex color buffer attribute. However, doing this is tricky since you want to color a specific part of the surface which means you have to know the exact range in the vertex data where the eye regions are defined.
A better and more straightforward approach is to import the PLY asset into Blender, apply the modifications and then export to PLY (or even better glTF). You can then import the asset with the expected color definitions.
You could solve this by manually adding a vertex color buffer attribute. However, doing this is tricky since you want to color a specific part of the surface which means you have to know the exact range in the vertex data where the eye regions are defined.
A better and more straightforward approach is to import the PLY asset into Blender, apply the modifications and then export to PLY (or even better glTF). You can then import the asset with the expected color definitions.