Tin is a relatively bright metal, so bright that we refer to it as a white metal, however all metals absorb enough light that their powders will always be a least somewhat grey. Tin is light grey.
Oxides on the other hand are very often very clear and thus totally white when finely powdered as they absorb very little light. Tin oxide is white.
If the powder is white then it isn't pure tin metal.
From what I googled just now traditional Indian medicine had a very different conception of 'purification' than modern Western chemistry. They start with the already pure metal and grind and calcine it with all sorts of herbs over and over again and end up with an impure metal oxide and then identify this product as the 'pure metal'. I suppose this could be intended as some kind of spiritual purity or just a simple physical misunderstanding or some combination.
BTW, until around 1780 Western chemists would have agreed. The Indians would have been seen as making dephlogisticated tin, ie removing the phlogistan from tin thus purifying it.
Anyways if the market is selling a
traditional 'purified tin' product and it is white then this all makes sense.
Now you aren't going to get a metallic tin powder by heating it with alum or herbs. You very well could get a substantially tin oxide powder by melting the tin with who knows what and crush and grind from there. It is certainly much easier to powder a brittle, crushable oxide. Potassium Alum might be able to hold on to its tightly bound waters of hydration long enough to provide water to molten tin to begin to oxidize it.
As long as you have grey you probably still have some metallic tin present. I imagine you will have to repeatedly crush and calcine it before you get rid of enough for it to be completely white.
Here is a paper that argues that such traditional methods make nano-scale tin oxide:
http://article.sciencepublishinggroup.com/pdf/10.11648.j.nano.20140204.13.pdf