You are using the Subkismet captcha, it doesn't support using different colors. The note you saw about color was for Recaptcha which has 4 themes you can choose from. In the next release of mojoPortal the Recaptcha theme setting will move from Web.config to theme.skin. Subkismet does allow you to override a few things including the instruction text but not colors.
See this new article I just created about styling/configuring captchas in mojoPortal.
Hope that helps,