The Joomla login button colors are defined in the sc_bootstrap.css file in the ".sourcecoast .btn-primary" style block around line 2577. Register button colors are defined at the ".sourcecoast .btn" style block
My suggestion would be to create your own theme file for CSS and redefine the colors for those two blocks in your new theme. For more instructions on how to create your own SCLogin theme, please see
www.sourcecoast.com/sclogin/documentation/themes