I have a graphite 6p and it's about as slippery as an HTC One M7. I however also use the Protective case by Adopted from the Google Store. It doesn't add hardly any bulk, and the case finish is matte on the back and has a slight texture on the edges. It definitely adds a better grip to it and is very close in color to the graphite finish.
While I understand wanting to use the phone without a case, smartphone OEMS generally focus on looks over functionality. A smooth finish looks better than a textured one, but that comes at the expense of grip. The only smartphone casing material that I've found that aided in grip was the soft touch material, like what you see on phones like the original Droid Maxx and the Nexus 7 2013, or a textured soft touch like the Droid Incredible 4GLTE had. Soft touch has a somewhat rubbery feel to it and so it adds to the grip. Materials like aluminum, glass, or smooth plastic may be fine if your hands are dry, but if you sweat at all, it actually makes the grip worse. And even textured plastic, while somewhat better than smooth, is still not as good as a case. The reason being the design ideology differences between the two. A phone is made to look good first and foremost, where a case is made to aid in grip and protect the phone from a drop.
Without having experienced anything other than my graphite 6p, I can't speak expertly on the gripability of one over another. But considering the only possible difference between graphite and frost would probably be that the graphite has an anodized finish where I believe the frost is painted. The painted finish may add a slight amount of grip (depending on how smooth the finish came out) but I doubt it could stand up to scratches as well as an anodized finish would. Of course it's bare aluminum underneath which is closer in color to frost than to graphite. Of course your best bet would probably be the aluminium model, since it's most likely the same color all the way through the material.