The browning can be due to all kinds of things, like residue from its production (ephedrine, for example, is notorious for burning). However it could be you're just using too much heat. Hold the flame further away from the pipe - you don't want to incinerate the meth, you just want to get it slightly past the point of melting then agitate the pipe to get a nice head of vapour. You might also try using less meth each time you put some in the pipe. Try only putting in what you plan to smoke over the next hour or so.