I Am A 49 Year Old Healthy Female. I Do

I am a 49 year old healthy female. I do not smoke and I do not paint my fingernails. I do not take any medications, but multivitamins daily. (Generic Centrum Silver, vitamin C, and B-complex in the am, and Ca at bedtime) For the past year I've noticed that my fingernails turn an Amber color as they grow out. My nail beds are fine, the nails do not turn amber until about halfway up the nail. Furthermore, each nail has a tiny subtle horizontal indentation about halfway up the nail in the center. This indentation does not go all the way across. At first I thought it was from my self tanning lotion, but I have not used that in 8 mos. . What could be causing this?