Between August 2005 and January 2006 I developed a PHP5 color conversion library which became the PEAR Image_Color2 package.
The impetus for this was trying to sort my Flickr favorites by color. After some experimentation and frustration with the RGB color model I did some research and discovered that I really wanted to be sorting by hue. So converting to color model like HSL would make it much easier. I spent some building a basic color conversion framework then polished it up enough to be published as a PEAR module.
The package supports conversions between the following color models: