There is a small thread on WR about how to talk about cars and stoplights, although it only deals with a single aspect of it. I’d like to address some other important points because the subject has tripped me up before. Technically, one of the terms (farol) means ‘lighthouse’, but in modern usage, it has come to mean ‘stoplight’ as well.
Stoplight – Farol, Semáforo or Sinal de Trânsito.
Red/Yellow/Green light* – Sinal vermelho/amarelo/verde.
* – To express red or green, keep in mind that you can say ‘sinal fechado‘ (red light) or ‘sinal aberto‘ (green light). In basic terms, it means that the intersection is momentarily open or closed to traffic.