Darker sound is achieved if you use mouthpieces w/ no baffle or less...,common sa mga ebonite, hard rubber mouthpieces...(but not all..)
Use in classical, and some jazz music...
Brighter sound if you're using metal mpcs like Beechler, Arb, Dukoff ,most metal pieces have baffle... . .but not all ....
Think about the sound of David Sanborn, Eric Marienthal, Warren Hill as examples of bright tone...
Charlie Parker, Cannonball - for darker
Kenny G - too bright (soprano sax)
I'm using Dukoff/ARB on my alto sax -w/c is more of contemporary/bright tone
but on soprano - dukoff (bright) and a cannonball mpc (dark)
tenor - Custom link by Phil Barone (bright, for it was customized w/ baffle)
so if you plan on playing w/ a jazz, funk,rock band, you need a brighter mouthpiece but if you're playing classical or w/ concert band,even big band darker mouthpiece is highly recommended...