That's what I said!  ;  ) ...

*If you use a sphere painted with a blue gradient and use this sphere as a filter your scene will be blue, what is wrong!

*Yep, but I did'nt mean, ...err would'nt know how, to filter Global Illimination through each pixel of your sky image, especially with Poser's 8 Ligth Preview limit.  Rather the Sky image will have a sun in it somewhere, which would tell you where to aim your light into the scene from.