There is a lot to the selection of grinding wheels. Aluminum Oxide
comes in three or four basic colours - brown, blue and white. White
is more friable (each crystal breaks off more easily) than brown while
blue is in between. A grinding wheel consists of the material
(Aluminum oxide or silicon carbide), the grit, the hardness, the
structure and the type of bond (vitreous or resin).

The harder the material that you want to grind, the more friable you
want your wheel to be. When a grain becomes dull, you want it to break
off and expose a new sharp surface. If this does not happen the wheel
will glaze over and will not grind anymore. Thus white aluminum oxide
will be suitable for harder material while brown aluminum oxide will be
okay for softer material such as mild steel. Silicon carbide will be
suitalbe for non-ferrous material such as aluminum. Similarly you have
to choose the right hardness for the wheel and the right structure.
Too hard a wheel will tend to glaze if the material ground is also
hard. You will mostly use vitreous wheels for you grinding jobs.
Resin based wheels are good for applications such as cut-off wheels.

You may want to do a google search. I suggest entering a topic such as
"Brown vs White Aluminum oxide". You should come up with a lot of
information

Chip