I would not read much into the colour, it's probably just how the guy who made the 3D model set the surface of solid bodies to appear in his CAD package (e.g. CATIA uses a similarly suggestive grey by default). It isn't even textured with windows and other similar details - chances are this is a geometry straight from an engineer's work station and pretty much innocent of any deliberate artistic touches. Since the PAK-DA is more likely than not to be supersonic, there may be clues in there, but I would not count on it.
As far as the design itself goes, it certainly looks coherent, but I prefer Sukhoi's quiet SSBJ studies - the curved edge of that planar dorsal surface in front of the inlets looks like it could result in some interesting issues in supersonic flow. Then again, this is TsAGI and I certainly can't profess to know better than them