Problems After nrrd Export

I am very new to 3D Slicer. I started using it to 3D render the series of labeled DICOM files I have. It does a fascinating job in rendering them, and using the Segmentation Editor, I can give different colors to different labels. However, the problem starts after I try to export this 3D rendering.

When I tried to export it in the STL format, the file has a size greater than 1 GB. Hence, I tried the nrrd and nii formats. However, I experienced a couple of problems

  1. When I import the file back, the quality in 3D seems to be decreased a lot. Why would that happen?
  2. The colors I assign in the Segment Editor seems to be lost. Is there a way to preserve these colors?
  3. I cannot find a way to set opacity. I looked at many other discussions, and although I can control the opacity in the 3D rendering made from DICOMs, I could not find a way to do it in the imported nrrd file.

I would greatly appreciate your guidance and support on any of these problems I have encountered.