The differences is miso can be attributed to a variety of factors, from the proportion of ingredients, cooking techniques and duration and temperature of fermentation. The reddish color comes from long fermentation times (or in cheap imitations, from dyes). The whiter miso’s are generally fermented for short times and often made with rice in addition to soybeans. Reds are generally saltier and whites sweeter.
Miso’s vary widely in fat contents (like most soy products) and generally rage from .25 to 1.5 grams of fat per tablespoon. Some varieties, like peanut miso, obviously have even more fat. Since it is rare that a dish has more than 1 tablespoon miso per serving, miso does not generally add appreciable fat to a dish. Many of the white sweet misos clock in at the low end so if you do want to use more miso, these kinds can be added to a dish (and since their taste isn’t as strong or salty, they do become many dishes in greater quantity).
