Wouldn't that usually hide the invisible character's shadow a lot more efficiently among the shadows of walls, doors, furniture, curtains and so on than before - as they all are a uniform bright pink now? I would indeed imagine the rypical medieval room - with a small window - to just become overall bright pink under this spell, with no nuances left but around the window and perhaps a single shaft of daylight.
You're right, and actually, I should have been more specific : I was trying to detect an invisible dwarf, and therefore, cast the spell with +1 magnitude in order to only affect shadows smaller than those of a human. What I was trying to do was to avoid making conspicuous other invisible magi of normal size... but I must confess we hadn't thought of the shadows of non-living objects!
Also the size of a shadow is not only determined by the size of the object or being casting it. And does a hedge cast the shadow, or its leaves? The bricks, or a wall?
Shadows are very tricky to deal with under Hermetic Magic -- because a shadow is not something, but lack of something (illumination). So you can't really target a shadow directly any more than you can target a hole (you can fill the hole, or dispel the shadow through light, but you are not targeting the "emptyness" itself). I'd guess you could target ambient light to detect shadows as lack of light, but in case of multiple shadows "in the same place" you would not be able to resolve them -- that place is lacking light, and Hermetic magic can't really "connect" to the causes of the situation, just like working on an empty room it can't really affect the people who walked out of the room making it empty. Does that make sense?