Many factors cause teeth to look dull and yellow. Intake of certain foods can discolour your enamel (the outermost layer of your teeth). On top of that, plaque accumulation on your teeth can prompt them to look dull and yellow.
The discolouration can be reversed with routine cleaning and whitening solutions. Occasionally, the teeth may also appear yellow because the hard enamel has been eaten away, leaving the dentin exposed. The dentin is the yellow bony tissue located beneath the enamel.