This gene has a lightening effect on eumelanin (black-based colors)
only.  It has no effect on phaeomelanin (red-based colors).

It is believed that the Brown Locus codes for an enzyme,
tyrosinase-related protein 1 (TYRP1), which catalyzes the final step
in eumelanin production, changing the final intermediate brown
pigment (dihydroxyindole) to black pigment.  SO, ALL dogs start as
BROWN and after the final step --- this directs the color to be black.

When brown (b/b) is expressed, it means that the final step in
eumelanin production has not been completed and the pigment
remains brown.  The brown color is not a genetic defect.

When the alleles are in the homozygous or heterozygous dominant
form of B/B or B/b, the color and pigment (nose, eye rims and lips)
remains (or directs the color to be) black.

When the alleles are in the homozygous recessive form (b/b), the
color and pigment will be brown.  This just means that the final step
in eumelanin production of changing brown to black did not occur.  
Phaemelanin (yellow/red [e/e]) is not affected.  BUT, in the e/e
colored dog, if the dog is also b/b; they will be either red or yellow
and will have brown pigment (nose, eye rims and lips).  The pigment
granules produced by "bb" are smaller, rounder in shape, and
appear lighter than pigment granules in "B" dogs. The iris of the eye
is also lightened.

Keep in mind that when speaking of colors, a dog isn't "simply" one
color or the other.  His color is being dictated by several allelic pairs.