The term hygrophanous or infrequently hygrophaneous, is rarely used outside of mycology or beyond the specialist terminology applied to distinguishing characters in mushroom field guides.
Used to indicate a mushroom's colour that notably "fades when drying."
The effect can be weak to strong.
To the casual observer, it appears to be a two-tone colour change, with the lighter shade being the opaque drier marginal tissue. Those people familiar will the species disposition can use the behavior as a kind of rain gauge, that shows the surrounding microclimate's ambient moisture changes - from fog or dew, and also reveals the invisible rivers of the near ground air currents. Drying time can be calibrated, the thinner marginal tissue acts as "the big hand", while central tissue, acts as "the little hand."
As hygrophanous mushrooms dehydrate
, the patterns and colors on the cap change, light penetrates the cap differently when the tissues are translucent with moisture. Another colour effect occurs when the colour of the underlying gills
show through, as the caps margin becomes saturated
. This effect is most significant in the genus Coprinus
-inky caps, where the gill's jet black penetrates the beige cap as a skirt of industrial bruises.
Next time you see a lawn mushroom see if you can spot this trait.
Any implanted references to wet T-shirts though should be more correctly to hydrophanous, though, that is probably the better term for what happens to a well wetted Coprinus, now that you mention it. Nod to tdent