Cherryheads seem to do that as they age. I'm not sure if humidity has that much to do with it. My adults all look 'dry' until it rains for several hours or I keep the sprinkler on them for a while. Then the color comes back.
The young ones don't display this as much and I sort of think that cherryheads just have that look. They are from a much drier and warmer area of Brazil than other types of redfoots so that may have something to do with it.