"Το φανάρι είναι κόκκινο."

Translation: The traffic light is 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'.

[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.

[Leon2018]

I used the plural for traffic lights even though I suspected that you would rule it out because you have a poor grasp of valid alternatives.

[jaye16]

If we had wanted to describe more than one light we would have used the plural.

For example, there might be one red light on the right side of the road, and one on the left in a large intersection. Or maybe two sets of lights in the same place - one for the straight lane and one for the lane turning right.

However, the sentence says "The light was red."

Therefore, we need the singular.