We have a sewer smell coming from the bathtub and the tub turns brown,What is causing this and how can I fix the problem??

The tub turns brown? Are you saying the tub's backing up when it smells? Back to you, Tom

I do not notice anything but the brown color and the smell, we have a septic tank but we have checked it is not full.the brown color goes away after I scrub it with comet

MaryJo,

Are you on a well? You're problem may be that if you're on a well, you may smell a sulfide smell and think it's your sewer, when in fact, it may be your water. If your water is really hard and has lots of minerals in it, it can cause your tub to turn colors. (I'm assuming your tub is porcelain.) Another thing that may help you clean it off is toilet bowel cleaner. Hope this helps.

We have a sewer smell coming from the bathtub and the tub turns brown,What is causing this and how can I fix the problem????

My take is that you have two separate problems. The brown stains sound like iron oxide,(rust) in your water. Are you on a well?
Sniff around and tell me where the sewer smell's the worst. Around the base of the toilet? The drains? Let me know. Tom