Think of a greenscreen like a metaphor of a really nice looking wall (could be a car park) that only has one colour and has perfect lighting, some people even just use white a4 paper attached to the wall with nice light stopping all shadows. Cause to pull that effect off there can be no shadows or darkness, The colour can be any colour but obviously people prefer green or blue for obvious reason but any colour will do.
p.s you dress your actor up in any colour except from the greenscreen colour.