The teeth are subject to caries (decay), caused by acid from bacteria In plaque, a yellowish film that builds up on teeth.
牙齿易成为龋齿（蛀牙），由齿面的淡黄色薄膜（齿斑）上的细菌制造出的酸而造成。
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And so with that in mind, I sampled a yellowish-brown color and on a new layer that was set to Multiply and 30% opacity and painted that color to the teeth with the Brush tool.