This idea is a mistake, a myth.
Underneath the enamel layer is a yellow and hard layer called dentin. In some individuals, this tooth enamel is thinner. For this reason, the yellow and solid color of the dentin under the enamel is seen more clearly. This does not indicate that the tooth is unhealthy, on the contrary, it is a healthy tooth. Due to bad oral care, the enamel layer becomes thinner and the teeth appear more yellow than the natural tooth color. This is the unhealthy tooth.