IBL mimics ambient light.  it's sort of a fake to reproduce the effect of light bouncing off of surfaces surrounding your subject.  iirc, most indoor light is actually ambient and not directly from light sources like light bulbs.  and it's why so many ceilings are white even if the walls are not- to bounce light back down into the room. 

the light gets colored by what its bouncing off of.  if something is reflecting red, you see it as red, and the ambient light it puts out is red.  the sky is blue, so the ambient light it puts out is mostly blueish. if you want to realistically light that scene, you need an IBL that takes into account the color and intensity of  light reflected by the environment around the girl.  that's why bagginsbill's tool Slinger linked to is so important.  if your IBL and environment don't match, it looks unreal. 

it doesn't have to be complex.  you can use bagginsbill's tool to generate an image for your Image Based Light (IBL) and match your 3d scenery.  or, since the IBL in Poser aren't very accurate, you can roughly approximate the ambient light in your scene by painting your own image for the IBL.  my suggestion is to just use the tool.  it's much simpler.