Corrosion is the phenomenon due to which te metals start to get converted into unwanted compounds due to the action of the atmospheric substances like air,moisture, no2, so2 etc.
rusting of iron- Brown. iron oxide.
corrosion of silver- black coloured coating.
there one more metal called pittal in Punjabi and Peetal in Hindi. it's corrosion causes deposition of green coloured layer in it. Sorry but I don't know the name of this metal in english.