> One theory is that much of the meth contains residue of toxic chemicals used in its production, or other contaminants. Even traces of certain chemicals, in a relatively pure drug, might be devastating. The sheer number of users is up, too, and the abundance and low price of P2P meth may enable more continual use among them. That, combined with the drug’s potency today, might accelerate the mental deterioration that ephedrine-based meth can also produce, though usually over a period of months or years, not weeks.

These are the theories mentioned in the article referenced in the comment you replied to. It might be the meth itself. Gin, whiskey, and tequila are different colors, and it's not because they are compositionally identical.