Customers ask me about shades of color on early US issues and how to identify them. You can find some cheap color guides out there for $10 or $20. They are a good guide to general colors on new worldwide issues. They are not a good guide for shades on early US issues.
There is one authoratative source for color on early US issues. In 1981, Roy White published a series of books called "Encyclopedia of the Colors of United States Postage Stamps, Volumes I-V". Volumes 1-V cover color and there is a Volume VI which covers paper and gum on early US stamps. Each Scott number is shown with various shades of color. The color plates are outstanding and are a quite helpful in identification. I believe only 500 copies of each volume were printed. So they are in demand.
You can find a set of the five White books from a philatelic literature dealer for about $700. A full set (with Volume VI) runs about $1100 (Volume VI is more scarce and runs about $500 by itself). You might be able to find a set on eBay for a little less. I've personally dealt with Jim Lee, Phil Bansner, and Leonard Hartmann. All are quite reputable dealers in philatelic literature and have some or all of the White books in stock. If you need their contact information, let me know.
If you are interested in shades on early US issues, the White books are indispensable and worth every penny. I don't have endless hours to identify stamps. But I'll make you an offer. If you have six (6) or less US stamps that you are wondering about the shade, I'll use my set of White books to identify them for you at no charge. All you need to do is send me the stamps identified by Scott number and a SASE for their return. This is a one time offer per customer. And it's a way for me to give a little something back to you, my faithful customers.