Prismwake Merrow

Color-changing on a body is a rare effect, and bolting it to flash is the design choice that gives this Merrow its teeth. The trigger reads as filler until you notice it can fire on the opponent's turn, on the stack, in response to a spell or ability that cares what color something is. A Merrow held up over a combat step can turn a creature a color that walks it straight into a protection ability or a color-keyed removal spell already on the stack; it can fix one of your own permanents around an enemy answer that names a specific color. The "until end of turn" clause and the "color or colors of your choice" wording matter as much as the flash: you assign the exact colors (one or several of the five), and the change unwinds itself before any lasting consequence sets in. It is a narrow tool with a wide list of one-off applications, the kind of effect that lives or dies on the rules text it can manipulate rather than on its 2/1 frame. Most of the time it just lands a marginal body at instant speed; in the moment the color of a permanent decides a combat trick, a protection clause, or a targeting restriction, it is the cheapest answer to a problem most decks have no answer for at all.