Walt's methamphetamine becomes blue when he switches from pseudoephedrine reduction to reductive amination. The blue color is apparently a plot device, introduced by the show's writers to make Walt's product visually identifiable. Although blue methamphetamine, sometimes called "smurf dope," exists in the real world, 100% pure methamphetamine would appear as colorless/white crystals. The blue color might result from impurities formed during the reaction, but despite its light blue coloration, Walt's product is highly pure. The episode "Box Cutter" opens with flashback to Gale giddily setting up equipment in the laundromat super-lab. He tells Gus that he has the sample he asked him to analyze saying it is "quite good." He then lets us know it was Walt's product: "I can not as of yet account for the blue color." Gale goes on to guarantee Gus a purity of 96 percent for his own product. Walt's sample was 99 percent pure and "maybe even a touch beyond that."