Could this yellow thing at the back of my teeth by tartar or just the tooth?

In short, yes: Please have this checked by your dentist to make sure all is well.

Where ?: If it is behind the bottom front teeth, then it is probably calculus (tartar). Also frequently found on sides and between top back teeth. See your dentist for a cleaning and exam.

Very common: The most common sign of tartar is a yellow or brown color to teeth or gums. See your dentist for treatment.