^ How do you know it is sewage? Did you taste it?
Seriously though my sink backs up sometimes due to problems with the garbage disposer and although it looks similar in color to what was in your tub I can promise you it isn't sewage.
The smell. Health Department also confirmed it to be sewage. It's not really something that 'backs up' really either. I'll just be on the computer, hear a bubbling sound, go look in the bathroom, and it's filling up with sewage. So yea.