Amphetamine freebase, when purified, is a crystal clear product with a very distinct but strong smell usually.
You can test the purity of your freebase by carrying out a boiling point determination for example but the smell and colour can be a good enough indicator I think.
A yellowish appearance is not always too bad but it comes from impurities like unreacted P2NP or P2P for example, the unreacted/unfiltered P2P/P2NP can also cause a fishy smell in amphetamine, which is often associated with a high product quality but is rather a sign of impurities. There can also be other impurities left, you can check the following links to learn some more interesting things about amphetamine:
h
ttp://bbzzzsvqcrqtki6umym6itiixfhni37ybtt7mkbjyxn2pgllzxf2qgyd.onion/threads/amphetamine-testing-protocol.349/
h
ttp://bbzzzsvqcrqtki6umym6itiixfhni37ybtt7mkbjyxn2pgllzxf2qgyd.onion/threads/amphetamine-purification-by-acid-base-extraction.788/
h
ttp://bbzzzsvqcrqtki6umym6itiixfhni37ybtt7mkbjyxn2pgllzxf2qgyd.onion/threads/amphetamine-salts.361/
In case of your freebase being very impure, you can purify it with a steam distillation, it is saver than carrying out a vacuum distillation:
Steam distillation:
http://bbzzzsvqcrqtki6umym6itiixfhn...le-steam-distillation-of-p2p.8040/#post-41494
Greets,
Gordon