+100 for this as well. Was just googling to see if anyone had found a solution for this and found this thread. Desperately need this for a current project.
Would be nice to simply have an option when creating the color picker field to choose HEX or RGBA format similar to how you can select options for how data is returned for images etc.
Would think this should be a pretty quick fix for Elliot.
Edit: For now I am just going to use a PHP function to do it: http://weblizar.com/convert-hex-code-rgb-code-php/
