- The theme doesn't support color picker
Not all themes support the color picker. Check the theme's documentation to find out.
- Color module is not enabled
Make sure to enable color.module on Administer » Site building » Modules (admin/build/modules).
- File download settings
The color picker is incompatible with the Private download method and will only show when the download method is set to Public - files are available using HTTP directly.
Visit Administer » Site configuration » File system (admin/settings/file-system) to check the download method your site uses.
- PHP's image library GD is not enabled.
Check your status report at Administer » Logs » Status Report (admin/logs/status) to see if the GD library is working on your server. This is needed to actually create the images. If it is not, you need to install it or get your host to do so for you.
- You still see the Color scheme fieldset with a select box and a number of textboxes, but no longer the color wheel or the preview.