Studio umbrellas, white, or silver? And when to use shoot through (white umbrella)?

I'm using a Nikon DX, SB-910 and want to do studio portrait shoots. What kinda umbrellas should I use? And what size? Silver? Gold? White? And when to use shoot through (white umbrella)?

White umbrellas tend to make the light soft and a bit more diffuse. Metalicized umbrellas tend to make the light a bit harsher. Gold umbrellas tend to make the light a bit more yellow.

I would advise that you start with white umbrellas and only buy the other sort when you have clearly demonstrated a need for them. Actually, you can duplicate a metallic light source with aluminum foil wrapped around poster board if you don't need the harsher light all that often.