There are several federal agencies that play a part in the regulations that cover soap and cosmetic labeling.  Checking their websites will provide details.  Also checking the actual regulations in the Code of Federal Regulations is good.

Government Agency Websites

• FDA, Cosmetic Division
  • Key Legal Concepts: “Interstate Commerce,” “Adulterated,” and “Misbranded”
  • Color Additives Permitted for Use in Cosmetics
    
  • Aromatherapy
    
• Federal Trade Commission
• National Institute of Standards & Technology, Weights and Measures
• National Conference of Weights and Measures
• Environmental Protection Agency – Minimum Risk Pesticides
• United States Department of Agriculture (USDA)

Actual Regulations

Title 21, Code of Federal Regulations, Cosmetic Products:

• (700) General
• (701) Cosmetic Labeling
• (740) Cosmetic product warning statements

Rules and Laws

• Fair Packaging and Labeling Act 
  
• Food Drug & Cosmetic Act
• Uniform Laws and Regulations, 2006 Edition (links to full contents)
• Uniform Weights and Measures Law
• Uniform Packaging and Labeling Regulations