Here's a screen shot showing the sort of compositing setup you'll want to make.
There is an orange lamp shining from the top left of the shot, and a blue lamp shining from the top right. The floor plane is ignoring the blue light while the cube and cylinder objects are receiving light from it.
First, for this to make sense you need to know the difference between layers and render layers. A layer is a slot where objects can reside. A render layer specifies for a type of render to be made by arranging layers in a special way.
So that you can see which objects are on which layers:
(I can label the layers because I'm using the Layer Management add-on)
Here are the Mask and Exclude settings for the render layers:
Explanation of render layer settings
Main Render Layer: Layer 2 which contains the floor plane is masked out.
Floor Plane: Excludes Layer 3 which contains the blue lamp. Masks out Layer 1 which contains the cube and cylinder.
Note that it's not even necessary to enable the "Blue Spot Lamp" render layer for the blue lamp to have an effect as long as the layer it resides on is enabled. Disabling that render layer speeds up render time.
More info: Understanding Render Layers