Reflective Dye

Most dyes recolor. This one relights. Whatever you put it on picks up a metallic sheen that shifts with the light around you, sharper the closer you stand to a torch and strongest outdoors in daylight, where it locks onto the sun instead. Standing still it does not animate, so it reads as polished metal rather than as an obvious effect.

It works best on armor with large flat panels, since the shading needs room to travel across the sprite. Coat every slot at once and the character can start to look oddly three dimensional, which is either exactly the look you wanted or a reason to keep it to one piece. Strange plants are the only way to get it and the trader rolls a random dye from each one, so you cannot ask for this dye specifically and will end up with duplicates of others first.