Or, if it's possible, just change the captcha images without altering your current code.
The way hackers try to detect your letters is firstly by reducing your image (colour or not) down to black text on white using a "thresholding" technique (see the aricle). As the colours are removed the hacker hopes to have only clear well defined black text on white background. A robust, well designed colour captcha image will be such that as the colours are removed, so are at least parts if not all the parts of the letters removed. In this case the hacker never gets to see a distinguishable set of letters at the end of thresholding.
If the hacker can get your image to a state of distinguishable black letters on a white background then your captcha is effectively broken. To locate the letters the code just needs to look for the location of the black pixels. The article also discusses how poor attempts at randomly locating characters and poorly distorting them are essentlially useless.
To get a feel for how robust your captcha image is against thresholding texhniques to get black text on white background, you can use the thresholding features in something like Photoshop or Photoshop Elements (and probably other image editing applications) to see how much of the letters are still intact after thresholding down to 0.
Anyway, just some food for thought