As you might infer from PostmasterGS' post, what you collect (what you need to identify) will dictate the best color guide for you. There is no one-size-fits-all answer.
They run the gamut from inexpensive to unbelievably expensive. Subway had Michel's for $20 for the longest time; last time I looked it was back to $80.
Serious collectors of U.S. might turn to the multi-volume
Encyclopedia of the Colors of United States Postage Stamps, but a full set could run well over $1,000.
An option for less money is the
Specialized Color Guides for US Stamps by Richard M. Morris, which you may be able to find for $50.
So, it depends...
