I contributed a PHP5 color conversion package to the PEAR project.
It supports supports the following color models:

• CMYK - Used in printing.
• Grayscale - Perceptively weighted gray scale.
• Hex - Hex RGB colors i.e. #abcdef.
• HSL - Used in CSS3 to define colors.
• HSV - Used by Photoshop and other graphics packages.
• Named - RGB value for named colors like black, khaki, etc.
• WebsafeHex - Just like Hex but rounds to websafe colors.