One answer isn't going to fit all of your questions. In the early days the size and color of the Treasury Seal on notes varied greatly, as did the colors. In general they changed colors to thwart counterfeiters and to differentiate between series of notes.
To determine what kind of a note you have, look at the wording above the central portrait. The most common note in circulation today is the "Federal Reserve Note." You will find that the seal color is green. Following is a listing of other 20th Century US notes that you may find in circulation or at numismatic sales or events: