void mitk::LookupTable::GetColor(int x, double rgb[3])
internally simply calls
this->GetVtkLookupTable()->GetColor(x,rgb)
which expects "x" to be of type double. Probably a copy past error from the method right below
void mitk::LookupTable::GetTableValue(int x, double rgba[4])
Solution:
change method to
void mitk::LookupTable::GetColor(double x, double rgb[3])