Signifies that value of this property contains the main color of the Material.
This flag corresponds to the [MainColor] ShaderLab Properties attribute.
By default, Unity considers a color with the property name name "_Color" to be the main color. Use the [MainColor] ShaderLab Properties attribute to make Unity consider a color with a different property name to be the main color.
Additional resources: Material.color, ShaderLab: Properties, ShaderPropertyFlags.MainTexture.