Awesome, and since it would just be a hue transform on the existing sprite, should pretty easy to do [and I'm pretty sure I saw just such a color shifted piggy in the perf tests]
+1
and now for the negative side of this idea : they would have to make them soulbound to your street because I'd just about quit over all the whining because people would be taking the rare butterflies .... go ahead and try to tell me I'm wrong :(((
Different colored butterflies could give different amounts of mood and img when massaged and sung to, also different amounts of milk per harvest (ie a full stack for rarest of colors)