Is a land a colourless spell?
Lands are never spells, even if they have other types like Dryad Arbor.


  305.1. A player who has priority may play a land card from his or her hand during a main phase of his or her turn when the stack is empty. Playing a land is a special action; it doesn’t use the stack (see rule 115). Rather, the player simply puts the land onto the battlefield. Since the land doesn’t go on the stack, it is never a spell, and players can’t respond to it with instants or activated abilities.
  
  305.9. If an object is both a land and another card type, it can be played only as a land. It can’t be cast as a spell.