I've had an iOS modeling application "Voxel" for several years now. For the past year it has supported printing through Sculpteo. I looked at Shapeways but they didn't accept OBJ files, (unbelievably to me since they are practically the simplest format out there). I saw a post from 2011 saying that OBJ files were now accepted but that MTL files (the ones where you specify the color) are not.
I was wondering if Shapeways supports MTL files yet? I am not talking about textures or anything fancy, just the ability to specify the color of polygons.