You can do this all in Slicer.
Edit color table: Go to colors module, choose a color node that you would like to modify, click “Copy” button to create an editable copy (the default color nodes are read-only). If you choose a “color table” type of node (this is the most common) then you get hundreds of predefined colors and you can modify each by double-clicking the color swatch in the color column, which can be tedious if you want to make large changes. It may be more convenient to modify “Continuous” color node, such as “RedGreenBlue” because you can specify a few colors at arbitrary levels and the colors are interpolated between them.
Show scalar bar:
- Slice views: go to DataProbe module, check “Enable slice view annotations”, check Scalar bar / Enable
- 3D views: go to Colors module, clone the color table that you use for the volume, set the same min/max range as you have set for your volume, and check Scalar bar / Display scalar bar