Trademarks can be registered at the federal level (at the U.S. PTO site) as well as at the state level. State trademarks are only valid in the state where they are registered.
Patent, copyright & trademark : an intellectual property desk reference by 
Getting permission : using & licensing copyright-protected materials online & off by 
Trademark : legal care for your business & product name by 
The trademark guide : how you can protect and profit from trademarks by Tiffany Blue; Pantone color #1837
Prince's Paisley Park purple
Orange on the Reece's candy wrapper (Hershey Co.)
