tamneti

1. to darken; to grow darker
   Nebo je polako tamnelo pred oluju. — The sky was slowly growing darker before the storm.
   Kosa mu tamni s godinama. — His hair is getting darker with age.
2. (of metal) to tarnish
   Srebrni nakit tamni ako se ne čisti. — Silver jewellery tarnishes if you don't clean it.