The reason you want to extend it is so you can "override" the "create_captcha" function without altering the original helper in your CI system folder. Your CI system should stay untouched so that it's easier for you to update CI on future versions. Also, if something goes wrong in your extended code, you can easily revert back to the original.
- So create a blank file application/helpers/MY_captcha_helper.php
- Copy only the create_captcha function from system/helpers/captcha_helper.php and paste into MY_captcha_helper.php.
- Make the changes to those RGB values you want in MY_captcha_helper.php.