If you want to have good preview of your geometry in a viewport then I’d suggest using the Rendered mode and putting your geometry in the Custom Preview component and assign the material by name to it.
See for instance How to render/set tinted glass material in Grasshopper?
That way you don’t have to do your own geometry drawing and you can just use Rendered mode to view your geometry using the materials in the Rhino document. This works also for Raytraced.
Not sure how it would work with other display modes, though.