White Miso: This miso is made from soybeans that have been fermented with a large percentage of rice. The actual resulting color can range from white to light beige, and the miso has a definite sweet taste. It's best used in condiments like mayo or salad dressings, or in light sauces.
Yellow Miso: Yellow miso is usually made from soybeans that have been fermented with barley and sometimes a small percentage of rice. It can be yellow to light brown in color. This miso has a mild, earthy flavor and is better for general use in not only condiments, but soup, marinades, and glazes.
Red Miso: This is also typically made from soybeans fermented with barley or other grains, though with a higher percentage of soybeans and/or a longer fermentation period. It can range in color from red to dark brown. The deep umami flavor of red miso can overwhelm mild dishes, but is perfect for hearty soups, braises, and glazes.
Black Miso: Our information on black miso isn't entirely clear. Some sources say this paste is made entirely from soybeans, others say that it's made from soybeans fermented with hearty dark grains like buckwheat. Regardless, this sounds like the strongest flavored miso around (the descriptions remind us a bit of marmite!).
The depth of color with any particular miso can also tell you something about it's flavor. Generally speaking, the darker the color, the longer it's been fermented and the stronger it will taste. Both yellow and red misos can sometimes be labeled "barley miso," so check the actual color of the paste for an indication of how mild or strong it is.
What kind of miso do you generally cook with?