Circe warn Odysseus about several things. Most well known is the Sirens. She says no man has ever heard the song of the Sirens and lived. She also tells him of the moving rocks that no ship can get through. She also warns of a sea monster and a deadly whirlpool.
He plugged the sirens ears and avoided the Charybdis
Whatever colour they happen to be painted (one can not see the siren usually). I hope you are not confusing the siren (which makes the noise) with the flashing lights (which are blue).
sirens are deadly mermaids who lure sailors to their death with their amazing vocal ranges while they sing. because of this, the sailors are attracted to the noise and soon their ship collapses & most of them meet their untimely death.