"Το φανάρι είναι κόκκινο."

[helmad]

'light' is normal for traffic light

[jaye16]

I've included it as an alternative. Thank you.

[fazulakis]

Also, "the (traffic) lights are red" would be a common way to say it (at least in Ireland, and I believe also in the UK), even if only one of the lights is actually red.

[charnfield]

I don't think we would ever refer to traffic light in the singular in the UK. I said 'the traffic lights are red'.