We are insulating the external wall of a bathroom, and I know we should put green drywall. A neighbour gave us a couple of red drywall, saying it would do the job as well. Is it true?
Do you mean purple rather than red drywall? I see that you are from Spain, so I don't know if this answer will apply in your country - but here in the USA, the difference is that green drywall is moisture-resistant, but it is not waterproof. Purple drywall has superior moisture- and mold-resistant characteristics that make it much more suitable for use where it might be in contact with water.
Fire-resistant drywall (known as Type X here in the USA) is not water-resistant, so I don't recommend it in a bathroom - unless you "layer" the drywall. Install the red fire-resistant drywall against the studs, and then install a layer of the green drywall on top of the fire-resistant sheets. This will provide both fire- and water-resistance.