Red Acid Dye

A dark red ripple pushes outward across whatever you apply it to, and the dyed item stays visible in the dark without actually casting light. That last part is the useful bit, since it keeps a piece readable in caves where a flat color disappears.

The acid dyes do something strange to the Shadow Orb pet. Put any of the three on it and parts of the orb, sometimes the entire thing, stop drawing. If you want the light it provides without the sprite trailing you around, that is how it is done.

The Dye Trader hands these out at random for Strange Plants, so there is no asking for this one specifically. Turn in whatever you find and it shows up eventually.