There was a time when brown envelopes were cheaper than white envelopes. That is no longer the case. However, brown envelopes are still associated with mass mailouts by businesses and so carry a businesslike association. Brown envelopes are also less see-through than many white envelopes and thus are better to convey sensitive information.
Depending on the shade, brown envelopes may also be referred to as Manilla envelopes, so called because such stationery use to be made from Manila hemp.
Perhaps white envelopes are more your bag. Click here if so.