The best way to gauge purity is not by smell. The best way to gauge purity is to have it analyzed via Gas Chromatography or at least to use a testing kit which will tell you if its even MDMA
. As Komrade said, sometimes an acetone wash can remove unwanted adulterants, but likely it won't remove all, as the only impurities that will be removed will be those that are soluble in acetone. The procedure can be found here: Cleaning Ecstasy pills (or powder) via a simple acetone wash
. Recrystallization is a bit more difficult to do if one does not have laboratory equipment, although really all that is needed is a heat source, distilled water and a suitable container.
As MickleMouse said, often the color is a result of the manufacturing process. Some impurities may be left behind which affect the products color; MDP2P, from which MDMA is synthesized, for example, is brown so if some is left the final product might have a brown tint. This does not mean that color is an indication of purity though. Pure MDMA is off-whitish but any number of adulterants, impurities, dyes, etc. can affect the product color. In addition, MDMA usually comes in salt form, for example MDMA HCl, and different salts can affect the color.