Gel Dye

Whatever you dye ends up coated in the same wobbly translucent blue the Slime Gun leaves on a player, so armor stops reading as armor and starts reading as goo. It sits very close to the Blue Acid Dye, close enough that most people cannot separate the two at a glance, which is worth knowing if you already own one and were about to trade for the other.

It pays off in a full costume rather than on a single piece. A slime helmet over a body layer with almost no silhouette of its own turns you into a slime instead of a person wearing slime. Since the Dye Trader hands these out at random for Strange Plants, there is no way to ask for it. You keep turning plants in until it shows up.