The best resource on Miso that I know of is Shurtleff and Aoyagi's book _The Book of Miso_. It's quite a complex subject. The differences is miso can be attributed to a variet of factors, from the proportion of ingredients, cooking techniques, and duration and temperature of fermentation. The redish color comes from long fermentation times (or in cheap imitations, from dyes). The whiter misos are generally fermented for short times, and often made with rice in addition to soybeans. Reds are generally saltier and whites sweeter.
Misos vary widely in fat content (like most soy products) and generally range from .25 to 1.5 gram of fat per tablespoon (and from about 6%Cff to over 30%Cff). Some varieties, like peanut miso, obviously have even more fat. Since it is rare that a dish has more than 1 T 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 (.25 grams per T, 6-10%Cff) so if you do want to use more miso, these kinds can be added liberally to a dish (and since their taste isn't as strong or salty, they do become many dishes in greater quantity).
I find the dark, hearty misos make great gravy starters. Add a bit of water/stock, some nutritional yeast, spices and a thickener to some miso and voila, instant delicious gravy. A favorite miso-potato recipe follows.
~- Michelle Dick artemis@rahul. Net
On miso, you just have to try different kinds & brands. There are numerous styles of miso. Red misos tend to be more "savory" and white ones are usually more "sweet". Country-style (Inaka) is made grainier on purpose. If served as a sauce on veggies, probably it's not straight miso but mixed w/sugar and some rice vinegar. Btw a warning to vegetarians, some misos come "dashi-iri" which includes fish-based stock. I think the ingredients label in English will mention fish, but not sure (since I can read the Japanese, I don't usually check the English), so check the ingredients carefully. There is kombu-dashi (I even found some granulated packets) which is vegetarian, but most dashi is from bonito flakes (I guess it must be cheaper).