"piros" and "vörös". if you just want to describe a color you see, then "piros" is brighter, "vörös" is darker. there are some cases where you can't use them interchangeably, but you are less likely to make a mistake if you stick to "piros"
As I understand, 'vörös' is mostly used to describe something more emotionally charged, since it's etymology goes from 'vér' – blood. It's likely to be used in figurative speech, whereas 'piros' only describes the colour.