I believe it's the kind of alcohol used in the product. Or possibly the additives in the product. Or a combination of both.
I am thinking of this from the perspective of using Heet in alcohol burners for cooking. A while back I watched some YouTube comparisons of different types of alcohols used in these burners - which burned hotter, which burned cleaner, etc. You've got you methanol, your ethanol, your isopropol, etc. IIRC, yellow Heet was methanol. I don't know what any of the other Heets may be. They could all be methanol for all I know, and it's the additives that make the difference. I do remember yellow Heet being judged the best of the Heets for alcohol cooking burners - I believe because it burned cleaner, but don't quote me on that.