1. Callow means "young, inexperienced." It is almost always used in the context of "callow youth." Its origin is O.E. calu "bare, bald," prob. from W.Gmc. *kalwaz, perhaps from L. or Celt. From young birds with no feathers, meaning extended to any young inexperienced thing or creature (1580). (Online Etymology Dictionary)

2. Sallow typically means "sickly, yellowish." It is almost always used to describe a face or complexion. The word appears to be derived from the noun sallow, meaning a type of shrubby European willow plant that presumably has leaves or catkins of that color. (Dictionary.com) Strangely enough, the Online Etymology Dictionary shows the word is alternatively derived from Old English salo "dusky, dark": if I recall my first exposure to the word correctly, it was in The Lord of the Rings, and used to describe the complexion of orcs, and this dusky/dark sense seems to me to be the meaning Tolkien intended.

3. Wallow means "to roll about in some substance, such as water, snow, or mud." Usually it implies that this is for enjoyment, but sometimes it just means the subject is having trouble getting out. (Dictionary.com)

4. A mallow is any of various plants of the genus Malva, such as Althaea officinalis, the plant from which marshmallows were originally made.