Hmm, okay, our hypothetical Sorcerer hikes up his dress, I mean robe, while he wades through the mud. His legs and feet get coated in the mud. Once he's out of the mud he puts the robe back down, covering his muddy legs. Now he's entirely invisible again.
BUT, if he lifts the robe up again, would the mud be visible again? If he dismissed the invisibility and re-cast it, wouldn't the mud now be invisible since he was wearing it when the spell was cast?
If an invisible Sorcerer took off his clothes, they'd become visible again, but anything he put on would stay visible, right? After all, he's not tucking them into anything...
Fun with invisibility!