Knowing silas, he's trolling... if not it's probably just food coloring. I've seen blue, pink, and yellow meth that was dyed by the producer to "mark" their product ie. "Yo that pink meth is the shit!".
Are you sure the meth itself is rainbow-colored and it's not just light refraction...or visual hallucinations?
it can't be to much impurities cuz shit is so much stronger than the last batch