I’ve submitted a fix to VTK that allows setting PBR parameters directly (by setting interpolation mode to PBR in the VTK actor properties), this fix is included in Slicer Preview Release from tomorrow. I’ve also updated SlicerOpenAnatomy extension to take advantage of this and copy PBR material properties directly to the output file. Therefore, for more accurate reproduction of color and surface appearance in glTF files, you can set model display properties to use PBR rendering. When using PBR interpolation it is also recommended to enable image-based lighting, as described here.
I’ve also updated export of non-PBR materials so that the color is made more saturated during export. This makes the color of the exported models more similar to the original appearance in Slicer.