O fogo verde parece ser apenas para viajar pela Rede de Flu.

Para viajar pela Rede de Flu, usa-se o Pó de Fluorescente, que torna o fogo verde e baixa a temperatura para permitir a entrada no fogo.

“Incendio!’ said Mr Weasley, pointing his wand at the hole in the wall behind him. Flames rose at once in the fireplace, crackling merrily as though they had been burning for hours. Mr Weasley took a small drawstring bag from his pocket, untied it, took a pinch of the powder inside and threw it onto the flames, which turned emerald green and roared higher than ever. ‘Off you go then, Fred,’ said Mr Weasley.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 4 (Back to the Burrow)}}

Quando as cabeças das pessoas aparecem na lareira, não há menção de as chamas mudarem de cor.

“Amos Diggory’s head was sitting in the middle of the flames like a large bearded egg. It was talking very fast, completely unperturbed by the sparks flying around it and the flames licking its ears.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 11 (Aboard the Hogwarts Express)}}

Sirius estava olhando para ver quando ele poderia falar com Harry, o que seria mais óbvio para todos se o fogo ficasse verde quando ele aparecesse.

“Because I’ve just seen Sirius’s head in the fire,’ said Harry. He spoke quite calmly; after all, he had seen Sirius’s head in this very fire the previous year and talked to it, too; nevertheless, he could not be sure that he had really seen it this time … it had vanished so quickly …

‘Sirius’s head?’ Hermione repeated. ‘You mean like when he wanted to talk to you during the Triwizard Tournament? But he wouldn’t do that now, it would be too – Sirius!’ She gasped, gazing at the fire; Ron dropped his quill. There in the middle of the dancing flames sat Sirius’s head, long dark hair falling around his grinning face.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 14 (Percy and Padfoot)}}

Além disso, o fogo verde não é tão quente, o fogo ao redor da cabeça ainda é.

“Sirius!’ he said. Harry whipped round. Sirius’s untidy dark head was sitting in the fire again.

‘Hi,’ he said, grinning.

‘Hi,’ chorused Harry, Ron and Hermione, all three kneeling down on the hearthrug. Crookshanks purred loudly and approached the fire, trying, despite the heat, to put his face close to Sirius’s.

‘How’re things?’ said Sirius.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 17 (Educational Decree Number Twenty-four)}}

O fogo também parece verde quando a pessoa que viaja pelo Flu chega à lareira do destino.

“The Atrium was full of people; the floor was reflecting the emerald green flames that had burst into life in all the fireplaces along one wall; and streams of witches and wizards were emerging from them.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 36 (The Only One He Ever Feared)}}

Quando Dumbledore chega por Flu, o fogo em que ele aparece também é verde.

“The empty fireplace burst into emerald green flame, making Harry leap away from the door, staring at the man spinning inside the grate. As Dumbledore’s tall form unfolded itself from the fire, the wizards and witches on the surrounding walls jerked awake, many of them giving cries of welcome. ‘Thank you,’ said Dumbledore softly.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 37 (The Lost Prophecy)}}

O fogo é verde (e não quente) para a pessoa que faz a ligação.

Nós vemos os detalhes de como uma chamada de lareira funciona quando Harry tenta ligar para Sirius do escritório de Umbridge. A pessoa que faz a chamada da lareira usa pó de flúor e enfia a cabeça no fogo verde-esmeralda, mas o fogo na extremidade receptora permanece quente e laranja.

“Harry pulled off his Cloak and, striding over to the fireplace, found what he was looking for within seconds: a small box containing glittering Floo powder. He crouched down in front of the empty grate, his hands shaking. He had never done this before, though he thought he knew how it must work. Sticking his head into the fireplace, he took a large pinch of powder and dropped it on to the logs stacked neatly beneath him. They exploded at once into emerald green flames.

‘Number twelve, Grimmauld Place!’ Harry said loudly and clearly. It was one of the most curious sensations he had ever experienced. He had travelled by Floo powder before, of course, but then it had been his entire body that had spun around and around in the flames through the network of wizarding fireplaces that stretched over the country. This time, his knees remained firm upon the cold floor of Umbridge's office, and only his head hurtled through the emerald fire …

And then, as abruptly as it had begun, the spinning stopped. Feeling rather sick and as though he were wearing an exceptionally hot muffler around his head, Harry opened his eyes to find that he was looking up out of the kitchen fireplace at the long, wooden table, where a man sat poring over a piece of parchment.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 29 (Careers Advice)}}

Isso faz algum sentido - para ser capaz de enfiar a cabeça no fogo, a pessoa que faz a ligação precisaria usar o pó de flúor para que sua cabeça fosse transportada e não queimada.