If you collect used US stamps prior to 1890, did you know that stamps with various colors of cancellations are worth more than the same stamp with a black cancel? Some are worth considerably more. If you don't have a copy of the Scott US Specialized Catalog, you should consider purchasing a copy. If you're not worried about the latest catalog values, you can pick up a used copy of the previous years catalog at a considerable discount. The Scott US Specialized Catalog is worth every penny you pay for it.
For each major US Scott catalog number, two values are given in the main listing: one for an unused copy and one for a used copy. The price for a used copy is with a normal black cancel that is contemporary for the time period of the stamps use.
However, did you ever notice the section marked "Cancellations" that immediately follows the additional listings for blocks, plate blocks, etc?
For example, let's take a relatively common stamp found in most US stamp collections, Scott #147. This is the 3¢ Green George Washington produced by the National Bank Note Company. The 2004 US Scott Specialized catalog value for a normal used copy is $1.50. However, a copy of this stamp with a green cancel adds $55 to the value of the stamp. Why? Well, green cancels are very rare for this time period.
There are other cancellations which are also worth more. The ones in different colors of ink are the easiest to spot. But if you have the time and patience, you may find other cancellations which are also worth a premium. These include fancy cancels (usually of a geometric shape or some pictorial design) and cancellations on US stamps from foreign post offices (Japan, China, Phillipines, etc).
If you collect used US, do yourself a favor and see if you have any stamps from 1890 or before that have a cancel in a color of ink other than black. If you do, consult the catalog. You may be surprised at just how much some colors of ink will add to the value of an otherwise normal stamp!
Good luck hunting!