Why is my stamp a brownish color when the catalogue says it should be orange?

When orange stamps turn brown it is usually because sulfur compounds have attacked the ink and oxidized it.

Storage in some vinyl holders can contribute to this condition, and it can happen fairly quickly - some expensive stamps submitted for expertizing have even been affected while awaiting the authentication process.

It's similar to rust on metal, except other chemicals are involved.

If you do an online search for your stamp, or for other orange stamps such as US Scott # C13 or # C16 Special Delivery, you can often find examples of oxidized stamps being offered for sale. A reputable stamp dealer will always disclose this condition in the description and should discount the price of the stamp.