"Τα κόκκινα χείλη."

Translation: The red lips.

[deactivated user]

does this word have a singular form?

[D_..]

Yes, χείλος, and it is used either in anatomy or metaphorically to denote 'edge' e.g. το χείλος του γκρεμού = the edge of the cliff. For more examples see here.

[HellasCad]

Yes, it would be "το κόκκινο χείλος".

[uw6bKIKY]

Is this a very irregular plural? I've looked at the other comment, but still a bit puzzled. What would happen if I wanted to talk about one red lip and not one red edge?

[mizinamo]

Is this a very irregular plural?

No. Very typical for neuter nouns in -ος: το κράτος, τα κράτη -- το όρος, τα όρη -- το τείχος, τα τείχη -- etc.

(But το γεγονός - τα γεγονότα.)

[uw6bKIKY]

Ah, thanks. Got to try and remember this group. You keep telling me and I keep forgetting. Never mind - I'll be off the Greek soon with any luck!

[mizinamo]

Got to try and remember this group.

They're not all that common, to be fair!