I think you are referring to this interesting article by Michael Ullman. It's mainly about JG54 but right at the the end is a short article on one of Hartmann's "Yellow 1's"
http://www.clubhyper.com/reference/jg54colorsmu_1.htm
Edit: I went looking through my books and found another picture of the same aircraft taken from a slightly different angle. In that photo the camoflage looks very standard. The darkest fuselagecolor matches the darkest wingcolor perfectly - and they are both lighter than the RLM70 of the prop-blades. The different shades in Ullman's photo might be an effect of the sun being reflected much better by the fuselage than the wings due to the angle from wich the photo is taken. The sun must be pretty low since there's not much shadow under the left wing, so the sun is reflected fully by the fuselage but less by the wing.
Cheers! / Anders