If anyone in my classes is interested in portraiture, I usually tell them that the best piece of gear they can buy to improve their photography is a simple reflector. When they hear that a good one only cost about $25, they run out and buy one…. then, I know it's coming.
They come into class the next week and ask when to use each of the colors on their reflector. Let's settle this one. First of all, I need to point out that some reflectors have 5 different materials on them that can be removed and changed out to use a different material via a zipper. Other reflectors just have one color. I personally recommend the Westcott 5-in-1 reflector for size, versatility, price, and durability. I've tried other brands and they don't seem to be as durable in my experience.
The White Side
I use the white side most of the time, but I usually recommend that beginners use the silver side in most situations. The white side casts a very soft, clean light at the model and is useful in studio where flash is used, or when there is ample light outside like during a sunny noon-time shoot.
The reason I usually get beginners started off on the silver side is that beginners rarely put the reflector close enough to the model, so the white side shows no effect at all. Also, the white side is useless in low light situations unless it is extremely close to the model's face.
The Gold Side
The metallic gold material casts a very strong warm light onto the subject. Every time I decide to get fancy and try the gold, the subject ends up with a radioactive-looking gold face. I have seen some reflectors with a zig-zag white/gold side that I'd like to try, but I haven't given it a shot yet.
I have used the gold side with success only a few times for sunset portraits when the sunset was very yellow. Other than that, it is not overly useful in my practical experience.
The Black Side
The black side isn't a reflector at all. It's an anti-reflector. Photographers use a black reflector to cast a shadow on certain areas of the image. For example, if the lights are producing too even of a light on the model's face, a the black side of the reflector can cut out the light on one side to create more artistic shadows. If you want to sound like a photographer who is in-the-know, you should call the black side of a reflector a “flag” when used for this purpose.
The Silver Side
As I already mentioned, I recommend this side for beginners who never seem to realize how close the reflector should be placed to the model. The silver side is terrific for shooting in low light or where a strong light is needed; however, the light is often too strong for mid-day shooting unless it is feathered away. Many photographers use the silver side more than any other, but I personally end up using white more.
The Translucent Center
When you zip off the reversible material on a reflector, the middle of the frame is a translucent material called a diffuser. This side of the reflector is usually held directly above the subject to soften the sun's natural light. It will always go between the light source and the subject.
I use the translucent diffuser quite commonly. When I shoot sunset portraits on the beach, the wind is often too strong to use an umbrella or a softbox, so I like to use the diffuser as a convenient way to soften the hard light produced by my bare strobe when I'm in a pinch.