Yes I know what you are talking about. She does look dark overall, but the lighting is poor in the picture, and as I said, since she isn't in sunlight she does not have the typical coloring of a white mandarin hen. From the picture it appears that she has a buff coloring overlay, so it could be a slight variation. My white mandarin hens do have a solid white body with a lighter buff patch on their flight feathers.
Here is a picture of one of my white mandarin hens that I sold yesterday. Sorry that it isn't the best picture, but I remembered this discussion and quickly snapped a picture with my phone.