This is a modification that I did on the PHP Captcha Script originally found here. It takes color information and in order to change how the captcha looks. You need to choose colors that are not necessarily easy to decern from one another, but that are human readable. Usually you want to choose the noise color to be relatively close to the text color. The background color should be contrasting to the noise and text so that they stand out against it. The idea behing a captcha is to create "noise" around the text, so that a screen reader cannot pick out the letters, and automate the filling out of your form.
To Download my version of the original PHP captcha, Click Here.