The reason why you are seeing these strange colours when you go to buy a bag of speed is due to one of the following:

It isnt Amphetamine; its either another chemical (maybe inactive) or a chemical chemically similar to Amphetamine (like said above).
Or
Amphetamine or Methamphetamine is usually made in someones bath using sub par lab tools and ingredients, it is not uncommon to buy speed that has other bits and pieces left behind in the synthesis. E.g. Red (Red Phosphorous left over).