Yes I shot in a dark room with fairly dark surroundings, only with a weak print viewing lamp. However, wouldn't the dark patch in this case reflect also the light source?
Schematically speaking the difference is that we have a very large diffuse light source (outdoor shot) or a small spot light source (indoor shot).
I'm probably saying the same thing here, but just to test if I've understood; could it it be that the "gloss mirror" with a spot source on the side only reflects that light away down on the dark towel that kills it, and the reflection that comes at your camera is only a small diffuse component, while with in the outdoor shot light is coming from all directions so you then get more of those "first hand" (specular) reflections? Sealing off the sides and bottom and only letting in outdoor light from above or something would then solve the problem, but it would be a very cumbersome setup. So I guess the recommendation will be matte targets outdoor and glossy target only for indoor "lab" setups.
Essentially yes. Since the target surface acts like a mirror (glossy or diffuse) it mirrors the camera and surroundings. That will desaturate the darker patches most because they absorb part of the illuminant's color spectrum, but also reflect a large part of of the surface, plus the remainder of unabsorbed light.
I don't think the problem in the outdoor shot would be colored reflections, it was really only gray concrete and overcast rain and stuff, but just that the diffuse large light source somehow lowers the contrast of the target.
Okay, it then just desaturates patch colors by also reflecting gray surroundings in addition to non-absorbed patch colors, and thereby lowers contrast as well because the same amount of surface reflection adds a larger percentage to darker patch color reflection.
So if ambient light is abundant, it's safer to use a semi-gloss or matte target, and when illumination conditions are highly controlled (darkened surroundings, and even/balanced lighting outside of the reflection Angle of View), a glossy target may achieve slightly more saturated colors (because surface illumination is totally reflected away from the lens, and not a small diffused part of it).
Cheers,
Bart