Are you sure that it is pure meth? I don't have the big knowlegde on this subject, but I have heard that if the product is not cleaned up problerly, iodoephedrin and HI are left in the product. And iodoephedrine and HI will make free I2 after a while, that can cause the yellow/orange colour.
But again;
I don't have the big knowlegde on this subject
:)