[vtkusers] vtkLookupTable for scalars mapping in vtkImageData

[Pablo Lecumberri]

Hi VTK users,

I'm trying to change the vtkLookupTable associated to the output of a 
vtkImageMapToColors filter, but I can't find it.
I just want to change one color, leaving everything else untouched.
I've tried

vtkLookupTable *LUT = rep->GetPointData()->GetScalars()->GetLookupTable();

where "rep" is a copy of the output of a vtkImageMapToColors filter, but 
LUT is NULL.
Does anybody know where it is stored?

Thanks a lot!