Use pressure treated wood when ever you have wood in contact with the ground. Most is greenish in color resulting from copper which is used as a preservative. CCA lumber means chromeated copper arsenic. Wolmonized was one of the larger trade names.
EPA is in the process of requiring a phase out of all CCA treated wood by Dec. 31, 2003 in favor of alternatives. You will often find the alternatives which have a browner or redder color availbable in lumber stores under various trade names.
You probably want 4x4s for fence posts, 2x4s are not appropriate for most uses. What kind of fence are you installing?