"A way to distinguish Ditto from other Pokémon"

#PokemonGO: One of my field missions was catch a ditto, so I’d been clicking on pidgeys and rattattas for like a week. Today I clicked on a cp 218 pidgey and the ring was yellow which I thought was odd because usually it’s more of a green color at that low of a cp. I then thought “this must be a ditto” and it was.Thoughts?Edit: Just for reference I’m a level 34 via /r/pokemongo https://ift.tt/2kw3pax