Those '?' are images. To change the color, you'd have to edit the image file and change the color manually with an image editor. The file is /modules/mod_sclogin/css/img/glyphicons-halflings.png
Sorry that's a bit inconvenient. It's something we hope to make easier in the future.
If you have any other questions, comments or feedback though, please let us know.
Finally, if you haven't already, please consider leaving a rating and review for SCLogin, or our support, on the Joomla Extension Directory. It certainly isn't required, but is very appreciated:
extensions.joomla.org/extensions/access-...authentication/24054
Thanks, and good luck,
Alex