Discoloration of the nail plates

A student asked me the other day why her nails turned yellow. This is actually a question that is asked all the time.
In general, nail discoloration is caused by dyes, damage, drugs, disease, injury or illness. Basically, our nail plate can absorb a lot of substances as it is porous. For example, smokers will find that their nail plates turned brown because nicotine is actually absorbed. Intensity of the stain increases as nicotine accumulates. Other substances can also cause discoloration.
Dyes can also stain the nail plate which is why I always recommend using a high quality, professional nail polish. Some inexpensive brands cut corners & actually use large amount of dyes to reduce cost thus discoloration is more likely to occur. At the end of the day, you get what you pay for.
Some topcoats can also cause yellow or light brown stains when applied to the natural nails. Therefore, always use a good base coat as it also help seal & protect the plate from staining.
Damaged or injured nail plates also tend to be more absorbent.