Assuming that users access the different companies via different domain-names then, yes, you can make the login page look different for different companies.
You can do this by changing the script that generates the login page. You can find this script at
C:\ProgramData\Enterprise Distributed Technologies\Complete FTP\Login\index.jss
If you change the line:
response.writeUsingTemplateFile("template-" + request.uriParts.host + ".html", templateData);
then the script will build the login page using files called:
where company1.com and company2.com are the domain-names of two of the companies. Obviously you need to substitute the actual domain names.
All you need to do is copy the existing template.html file to those file-names and then customize each of them as required.
Does that make sense?