IRS & State Resources
IRS Circular E: This publication explains your tax responsibilities as an employer.
Publication 15-A, Employer's Supplemental Tax Guide: It contains specialized and detailed employment tax information supplementing the basic information provided in Publication 15 (Circular E).
Publication 15-B, Employer's Tax Guide to Fringe Benefits: It contains information for employers on the employment tax treatment of fringe benefits.
Colorado Department of Labor: Labor Laws
Colorado Department of Revenue: wage withholding