etoo, u better follow some good tuts, this is plain rubbish, no u have full authority to change the coolor of the render it doesnt matter, but try to be more realistic there, u cant simply change the renders color by adding effects andthat too in Differnce or Exclusion mode>.<

the rest is what others said
Try and make the render visible even though if ur bg is dark as hell.
Follow tuts here and u will evolve,
Keep Trying